Stop doesn't do anything that the round ending wouldn't, and kill removes the entity, which isn't what I want.
Exactol suggested modifying the particles to have a shorter lifetime, so I guess i'll experiment with that later.
As in, it deletes the entity from the current game and won't come back unless you reload the map. This is actually done by valve on occasion for light brushes that emit light for large windows in spytech areas. There's a logic_auto that onMapSpawn, will kill all of those light brushes to not be an issue to the entity count during the game.
That's weird because a new round does respawn entities. There are only few exceptions like env_sprite that doesn't get resetted by a new round at all.
It's easy to test, load a map of yours, type into console "ent_fire <entity-name> kill" and then "mp_restartgame 1". The entity should be back from the dead.
Buuut I don't think this is the solution to OP's problem, because when you kill a particle entity, the particles often are still there until they disappear on their own. This kinda depends on how the particle effect has been scripted, so OP can give it a shot, but I highly doubt it works in this case.