Yes you can. To do this, please log in to your administrator account and use the search boxes to search for the mentor you wish to remove. The account will appear in a results list. If there are students with the same name, ensure you locate the mentor account by checking the 'tag' or 'permission' column.
Once you've found the account, tick the box to the left of the name and then press the red deactivate button to the right of the name. Once done you will see a green success message telling you that the account has been updated successfully.
You can always reactivate any deactivated accounts by searching again for the mentor (ensuring you have toggled the status box to 'Inactive Only'). In this instance the button to the right of the results list will be a green activate button.