The remove disconnected devices button will remove all port connections that are older than the last seen date of the switch. If you've decommissioned the switch, it is indeed possible that some connections are not getting purged when you hit the button if they were scanned the last time the switch was scanned.
The connections listed on asset pages have a visible date though, and are ordered from newest to oldest. The top one should be the currently active switch connection in almost all cases.
It is theoretically possible to purge all device connections for the disconnected switch in question though, using a database script. If you want to take that path, please send a mail into support@lansweeper.com and reference the forum post, we'll be able to provide you with a sample script.