Dole out some punishment.

You quickly grab both of Kayla's wrists. "Don't...don't think you can get out of this by playing my cock, Kayla. I'm not Frank." Her mouth drops open, but she's not fooling anyone.
"You've been an important part of this company for many years. The last thing I want to do is call the police, Kayla." You hold her arms tight at her sides by the wrists, looking into her eyes very closely. "I'd be willing to let the $12,000 be your severance."
"Sever.....James...I can't lose this job. I was just getting to where I could go for my real estate license and we could start doing our own flips, our own projects."
"Well, you can kiss that goodbye, Kayla. Even if I'm willing to forgive the $12,000 which you should thank your lucky stars for, how can I trust you ever again?" You haven't let go of her wrists, but she hasn't tried to wriggle free.
"I'll do whatever you want. Revoke my access to the accounts. I'll just answer phones."
"Fuck that, and have to do your job for you?" Kayla is really squirming now, looking for a solution. "I'm not keeping you around to do less... I'm afraid I don't see a solution." You purse your lips.
She's really bawling now..."Please....James...." she blubbers. "I'll do anything. Whatever you want to prove you can trust me..."

A slow smile forms on your face...
"Good. That's what I wanted to hear." You pull her in close, spin her around and clasp her hands together behind her back.
"James..wha..."
You push her down onto your chair, face into the seat, and hold her in place with your knee. "Don't squirm. If you want to prove your trustworthiness, you will be obedient!"
Amazingly, Kayla stops wiggling. "....y.........yes...wh....whatever....you....say."
You pull a roll of duct tape from your desk, and tape her wrists. She's quietly crying, but you pay it no mind.
"the first step to trustworthiness is discipline, Kayla. You've done a very bad thing. And now you must be punished for it, to demonstrate why you should never do it again."
You march her over to the couch. Your father kept a leather couch in here for many years, and had probably fucked Kayla on it many times. How ironic.
You put her down on the couch, knees and face down. With cruel and mechanical precision, you hike up her skirt around her slim waist, and take down her panties, bunching them around her knees. You then pull a bandanna from your desk. It's old and ratty, but clean, and you proceed to gag her mouth.
"I'm going to punish you, now. Any time you'd rather just walk out of here, and never come back, just say the word...well, mumble the word, anyway." You're a bit shocked that she's even allowed this much to happen. She must be very ****. "If you want me to believe your loyalty, then you'll take your punishment like a good girl." You pull your belt from it's loops, doubling it over itself. Kayla's eyes go wide.

You rub the belt against her asscheeks, and she lets out a pitiable whine against the gag. You tease her pussy lips with it, rubbing against them, up and down, as you squeeze the flesh of her ass, not gently. You line up a swing, and then bring the belt across her cheek with a satisfying "crack!"

"NNNNNNNNN!!" she squeals against the gag, her eyes squeezing shut. A nice, angry red mark is already forming.

"Shh....sh...." you stroke her cheek with your other hand. "It's okay. It's supposed to hurt. That's how we...LEARN!" you punctuate the word with a backhanded "thwack" across the other cheek.

"Rrrrrrrrrrr!!!" she's sobbing against the bandanna now.

"Had enough already? Unable to prove yourself after only two strikes?" You kneel down to bring your face close to hers.

Slowly, she shakes her head.

"Still with us, then?"

She nods, slowly, and sniffles.
The pattern continues for about six strokes, until you notice a wet sheen on her pussy. You pause to probe two fingers into her tight snatch. Kayla moans, a very different pitch from her earlier squalling.
"Oh...you bad....bad....bad girl. You like this, huh? You got caught on purpose, didn't you? So that Master James would punish you like you deserve?"
Her eyes roll wildly, angry and shamed, and lustful. You pull the gag from her mouth roughly, which leads to a fit of coughing.
"Is your punishment over, bad girl? You ready to give up and leave this? Just say the fucking word, Kayla." She sniffles again.
"No...."she's looking into the middle distance. "P...punish....me more......"
"I'm sorry. You're going to have to SPEAK UP!" You jam two fingers deep into her pussy, and smack the top of her ass, near the tailbone, with the flat of the belt.

"PLEASE!!! PUNISH ME MORE, JAMES!" she was gurgling from trapped spit and mucus.

"Yeah you'd like that wouldn't you." You pull your fingers out, and bring them up to stick them down her throat. "You'd love Big James to take off his pants right now, give you the punishment you really deserve, huh?"

She nods..."I deserve it, sir." Wow. you can't actually believe how depraved this girl has become in such a short span. She really is **** to keep this job, and to be forgiven for her thievery.