Dungeon spawners don’t stop spawning

I found a spider dungeon while mining and quickly put a torch on the spawner. The first spider that spawned didn’t attack me, so I just went to the chests. I started to collect the loot when another spider spawned and attacked me. I killed that spider, and the one from before just in case. I put a second torch on the spawner and left, but as I was leaving a third spider spawned.