This is overkill, but you can choose the methods that work for your situation.
Do you have access to the Options Menu?
If you don't, you will want to uninstall TeamViewer and watch for the checkbox to remove settings also. When you re-install you should have access to options.
In Options, is there an Account Assignment?
On the General Tab, toward the bottom-right, there will be a statement if you are assigned to an account. If your are, try the remove button. If you can't, you will want to uninstall\re-install.
Change your passwords.
In Options > Security Tab, reset the personal password. Look for blue text that says "Manage additional Passwords..." and delete any entries. (This is only present if a personal password was previously set.)
In Options > Security Tab, adjust random password strength. If you don't need anyone to access your computer this can be disabled (no random password)
If the old employee still has an account on your computer, set Windows Logon to Not Allowed.
Check black and white list. Remove any entries from white list.
Disable incoming connections.
In Options > Advanced > Advanced settings for connections to this computer. Set access to suit your needs. Default is Full Access.
Hide online status.
In Options > Advanced > Advanced settings for Computers and Contacts. Check Hide online status for this TeamViewer ID.