Hi,
It does not matter how you remove the session, either you change protection to none or remove the session. The catalog information is kept in IDB until purged. The purge is running daily during omnicheck (see global for the defaults at highnoon or 12:30). However, obsolete filename version for a server are purged only daily when limit is reached. So I recommend to purge your IDB on a regular basis (depending on environment every 1-3 months) and with -force option for the filenames purge.
The good news: after you migrated to DP 8.XX or 9.0X, the purge is no longer required, instead in PostgreSQL the autovacuum fasture will do the job.
Best regards
Daniel