Block or mute someone, then actually write down why. A real app.bsky.graph.block or account mute, plus a dated, searchable note — because six months from now you will not remember.
Heads up: your notes are ordinary atproto records on your own public PDS repo — like your block list itself, they're technically world-readable (nothing here is end-to-end private). See the lexicon. Don't write anything you wouldn't want the blocked person to read.