I believe beginning with v5.02 the softdeletion is marked in the table userportals. Softdeleting will  as well harddelete the userroles in table userroles.

For the harddelete of a user I figure that first the aspnet_users and the aspnet_membership records will have to be deleted then the record in users, which should pass the deletion to relevant dnn tables. The ASPnet_users seem to be joined on the username/loweredusername, so it would be kind of hard to delete the records in users first.

When we do a harddelete, we only delete softdeleted users, which we first write (trigger) into a table when they are softdeleted. In that way we keep the userinformation of all harddeleted users.

We are aware, that any update of dotnetnuke might break that system (as 5.02 did compared to older versions) and that there might remain harddeleted userids in tables of other modules and that we have to pay attention to not delete userids which are registered on several portals.

We just setup an environment for that and for now it works. I am very curious about other opinions or risks we did not consider yet.

Chris, I am really glad to have someone to discuss that matter.

What we found is that executing a softdelete in dnn will not only set isdeleted in userportals but delete the corresponding userroles. If there are any roles left to the userid, they will belong to another subportal!? It might be an idea to do a count on userroles und to only proceed the harddelete if there isnt any role left!
I think there is no need to delete the userroles or userprofiles cause they will be automatically deleted if you delete in users.

Anyway, I am a little afraid of what will happen if dnn changes the dbprofile again, as done with 5.2. Before 5.2 isdeleted in userportals wasnt used. If you have an idea where to quickly pick up what is changed in releases concerning db structures I'd be very happy.