That person can keep a free version of their TeamData account for their own use, but you can stop access to all team data shared with the account.
Just search for the user or click on their username, and you will see all the data shared with them from your account. Then click “stop access” on EACH Gem. Every user in the company who has shared data with the employee will need to do the same.
In the paid TeamData for Business version, there are additional admin controls that make this easier and ensure that all company data is protected and backed up.