I don't know of any way to 'inactivate' a license on a target machine you don't have direct access to.
Plus, once installed, the subscription is actually part of the user-account login.
So you could, I guess, have an email/Adobe account like 'contractor1@mycompany.net'...and tell the contractor to register their installation with that account...but then, it's up to the contractor to sign-out and de-register their CP install when the contact is complete.
If you have trustworthy contactors who will reliably do that, that may be a solution.
If it's random folks and you have any concern about them continuing to use the login/subscription after the contract is done, then I wouldn't do that.
I'm not terribly familiar with the 'team' features though, so maybe there is a way to manage subaccounts...
However, the *better* solution may be to just put the entire software responsibility on the contractor themselves.
They can subscribe to Captivate on a monthly basis ($30/month):
Buying guide | Adobe Captivate 8
So if they don't already have Captivate, tell them to subscribe to it and include the cost in their monthly invoice... fold it into the price of contracting (what's $30 a month in that sense? maybe a half hour to hour of contactor's rate?).
The only caveat I'd add to that is, I don't know how easy it is to *cancel* a subscription. Some folks had it easy, some folks not. So you may want to check on that process, and be sure your contractors are clear on it...as once the contract ends, you're not going to pay for their monthly subscription anymore 