In the case when your colleague leaves the company, you can easily transfer the license to the new replacement.
To delete a member, go to Settings > Team and licenses. Hover over a user and select three vertical ellipses on the far right, then select Delete from the dropdown. If this person is a member of multiple workspaces, you can delete them from all workspaces at once. To do so, check the option "Remove from all workspaces".
You can CC another user from your account on the removed user's documents so that you don't lose important notifications when the document is signed. Then select Invite new user on the right. Type the email address of a new user and choose a Team Role for them. Once done, this person will receive an invitation to the email address and need to accept it.
Please refer to our Help article for more details.