dataset user access

Hello,

I need a idea to solve the following problem:

I have a table which datasets are owned by th pg login user. If the pg user is deleted, the owner of the dataset should be set to null, that means that only a super user can access to the dataset. If the username is renamed the owner of the dataset is also renamed.
I have found, that I can not setup a constraint to the pg_auth or to pg_user. The next problem I see, if I delete a user and add a new user with an equal username, the new user get access to the datasets of the old user. Can I create a reference from a table to the pg system table structurs, so that I get a "unique" user id and if the user id is removed, the field in my table is set to null?

Thanks

Phil