[mttjj]

Fastmail has first-class support for exactly what you want. Settings > Users & Aliases > + New Alias > "Show advanced preferences". There's a checkbox that says "Disable - Reject (bounce) all mail sent to this address (disable the alias) Disable a specific address when you have a wildcard (catch-all) alias."

[beeboop]

thanks - i didn't realize i could make an alias after the fact and then disable. appreciate it!