In the Show options area, change the drop-down in the Milestones box (fourth from the left) to No Milestones, then click Go.

It's not a top priority right now, sorry. For now, if one of your old threads pops back up as a milestone and you don't want it to, one option would be to repost the OP as a reply at the bottom of your thread, and then self-delete the OP. That way the OP is preserved in the thread, but the thread will be locked and will not milestone. It's not exactly the most elegant solution but it should do as a temporary workaround.

Alternatively I could lock your thread if you want the structure to be preserved. I would post a reply at the bottom noting something along the lines of, "Locking at the request of the OP."

And you're right that this isn't - and shouldn't be - a priority.

I did almost reply to a Milestone post over a year old, a week or two ago, but I noticed the date in time. And people can always edit the OP as I did to explain it's old and no longer relevant. But I really would like to completely stop replies in that thread, and locking seems like the best solution.