you can find this in the standard Mastodon web UI, under Preferences, Follows and Followers. Set Account Activity to "Dormant", sort by "Last Active". Select all then "Unfollow Selected Users". Change relationship to Followers and repeat.

Don't accidentally hit "Remove all followers from the selected domains"...I don't really know why that deadly action is in that particular spot.

This also purges a lot of spam accounts that usually don't sustain their crap beyond a day or so.