Just to be clear:
@thorsten imo @dnutan 
described the best possible solution.
This is because Roundcubemail (still) has the nethserver-mail package defined as mandatory in the repository comps group (see), However this particular misbehavior can be a middle ground to improve (not solve) the package removal. If packages exclusively known to belong to a module like nethserver-pbx are listed in the group they will be removed. This does not remove their children but i think it is doable (at least) to make sure a service is really shut down.