Hi Toby ,
Generally To remove a user from Enterprise dashboard we Login to Marketing Cloud------>Adminstration--->>Enterprise Dashboard----> Products--->Adobe Target----->Select Target product group to which user belongs ---->Select user to delete .
and the user is deleted . If you have done the same process Please logout from your Adobe Marketing cloud account and login again to Adobe Target and check to see if user is still show in the user's list for Adobe Target standard.
If the problem still persists . Please do let us know we will further debug the issue at our end.
Thanks & Regards
Parit Mittal