Well, did you kill the goblin rulers?
When that happens, no more gobbos (from what I remember)
Hmm... That is a possibility, do they ever come with a siege or squad? And if so, how would I go about subduing them without killing them? If I cage them do I still get to experience sieges and ambush squads? It's not like I want them to stop ENTIRELY, I would just like to be adequately prepared before facing 6 ambush squads each possessing 10 goblins.
When your fortress is wealthy enough and has repelled a load of sieges, the local leaders will show up, and eventually the goblin king (who will sometimes be a dwarf, apparently). As far as I remember they do retreat when wounded or if you pulverize their forces, but it's really hard to prevent the local leaders from dying.
Also they tend to be Elite Marksgoblins/Swordmasters/Hammerlords, so if you intentionally allow them to survive, you're risking losing a load of dwarves (at least until you have champions).