Have a look at your by-laws and see if there is something there that you can use – maybe something about disturbing the peaceful enjoyment of the lot or dumping rubbish on common property. If there isn’t one, draw one up.
Then tell them that the committee has had enough of their behaviour but ask them to come to a mediation – your local Community Justice Centre will organise that – to thrash out their grievances once and for all.
If they refuse, tell them that if they continure their bad behaviour, you are going to start responding in kind.
If they still behave childishly, start hitting them with every breach notice you can think of. Set up a cheap security camera to catch them in the act.
A few $220 fines should focus their minds a little.
A few years ago I recommended the use of a matchstick and superglue in a repeat offender’s lock. Of course, now that I am a responsible strata citizen I could never possibly suggest such a thing for the lock on their letterbox … even if it did work.