Having this exact same issue. It's not game-breaking or anything, but it is kind of irritating. I'm going to see if a Save As does anything to remove the effects following my household even after they've moved to a new lot.
Okay, after digging in a bit, I found a hidden trait on my sims that were active when I added the lot trait from the event, and it does seem to remain attached even after that household has been moved to a different lot. The trait is r3event_thrillseeker. I removed that using cheats, and the buff finally stopped re-applying.
It seems to be a little bit like how once a sim unlocks all their discovery traits, even if you use a retraiting potion to remove them, a hidden trait remains which prevents new discovery traits until you manually remove the hidden trait yourself.
