Generally speaking, if an employee leaves your organization, you can suspend their Dynamic Signal account, and that's the end of it. Occasionally, an employee may return, try to sign up to the program again, and be told the account already exists. In such a case, you can unsuspend the member and reactivate the account.
In order to find the account, however, you will need to make sure you are searching with the Suspended Member filter enabled. Check the box shown below, then search for the member in the Member Search box.
Once you have found the Member, click their name to go to their Member Details. Click the Settings menu on the left, scroll down and uncheck the Suspend Account privilege. Then scroll to the bottom of the page and click Save.