1) How are you determining if the registration has lapsed or not?
2) in the record for the thread, you should have a column that marks it as being locked or not. make sure that the recordset that returns the thread includes this status column, and write an if statement around the reply button to only show if the status is not locked.
3) Security assist will have an access rule for logged into <your users table>. apply that access rule to the 2 forum pages that should be locked.