About half a second after you turn on your scanner press 0 and then press 1 and then press Enter.
This will erase all talkgroups, talkgroup systems and conventional frequencies. This will also reset all Global Settings back to the factory default if you changed any of them.
This is from page 94 of the PSR-500 v1.3 owner's manual:
0, 1
Destructive, clears working memory and
resets all Global settings to factory
defaults. Does not affect V-Scanners.
Shawn