Disconnect with worse signal quality than -80dBm:
/caps-man access-list add action=accept interface=all signal-range=-80..120 add action=reject interface=all signal-range=-120..-81
since RouterOS 6.42 you can add a time limit:
/caps-man access-list add action=accept signal-range=-80..120 allow-signal-out-of-range=00:00:10 add action=reject
→ every client with less than 80dBm signal quality gets disconnected after 10s.
Sources: