Need to remove list of user from all associated groups

If you have the People Functions plugin in stalled, this can easily be accomplished.  You can create a process that takes in a single User value, and retrieves the list of their groups with fn!getgroupsformemberuser(), then run a Remove Group Members node as MNI, running once for each group in the listing against the User parameter.  A parent process model will call this sub process also as MNI, once for each user in your list.  

Note this will only apply to groups these users are Direct members of, if the group membership is defined by a rule, they will not be removed.  Assumptions are that both the list of users and groups per user are under 1000 items, otherwise a looping configuration would be used.  

OOTB, there is a little more setup involved.  Unfortunately the online documentation does not go back past 2019, so I cannot confirm which functions are available in 18.3.  I would also strongly suggest upgrading to a more recent version of Appian.