That depends. What software did you use to give them access? If you don't know the name, describe step by step what they had you do to give them access.Most, if not all, reputable remote access tools do not have persistence; they require acceptance from you every time the remote user wishes to connect.
All the remote user needs is username and password of the OP, and the OSwould never be the wiser.
The first thing that happens on all machines I manage, is disable Remote access, then install Team viewer, using a strong random password for inventation, that changes after each time you allow a request.
That way no one with previous access, can gain access again with the previous assigned password.