How can I enabling change of joinable usergroup, so users can't switch?

[Scubado]

Besides of the usual usergroup (registered users), lets call them members, I've made four specialised usergroups (A,B,C and D) Members can join one of them - our members can only belong to one of them- , which give them access to their own groupforums. So far so good, no problem. Notice, we don't know to wich group they belong, therfore they have to make the 'choice' themself.

But now I want the users to chooce for only one usergroup -instead for all- and stick to their group, enabling to choose for another usergroup once they have made their choice (request) and Only administrator can change it.

In the configuration I made now, they can send joinreguest for all groups and request to change of group.

In adition I would like new members to choose on registration for their specific usergroup.

Does someone know a way to arrange this?