Sounds like you have two things working against you (besides the obvious low-lifes who don't respect other people's property): The availability of useful things and the potential appearance that the boat is abandoned if it rarely moves.
Could you replace the halyard with a monofilament runner or two? If you leave it and they take it, it will be expensive and difficult to replace.
Mike
I was thinking of just pulling the halyard all the way up to the sheave (right now it's shackled to the mast base), coiling up the line and stowing in some sort of canvas bag attached to the mast (so the line itself is out of sight).
But knowing me I'd hoist the mast and fasten the forestay before I realize I didn't return the halyard shackle back down to the mast base
