When I put a tag on a post, Tumblr saves that tag. Meaning, when I start making a tag on a different post later, it lists suggestions for autofill from previous tags I’ve made which so far have all been saved. How do I make Tumblr stop doing this? I want to delete some of the tags which I think are already useless.
For the tab suggestions you'll have to go back to previous posts and remove them manually. Once that's done you'll be able to get the autosuggestions to stop.
One way to check is to go to
yourtumblr.tumblr.com/tags/tagiwanttodelete and make sure none are there.
However, popular tags used by other Tumblrs will still show up in the suggestions.