you should not use references in your module to core tables, i suggest to use triggers instead.

Generally spoken, modules shall reference core tables as less as possible, ideally all the dependency should be made in business layer, using events like "user deleted". whereever this is not possible, placing a trigger can enforce referencial integrity without relying on the structure of the table - only on the existence of a row, e.g. by adding a delete trigger, that deletes the same row in your table. To be honest, i am not a fan of physically deleting rows from the users table, I would prefer, if the user is only marked as deleted, this allows all referenced data to be used: imagine, the user has previously created posts in the forum - shall they get deleted, or his name be replaced by "anonymous"?