lol, +1 for MTTs suggestion
thats pretty rubbish, i dunno how seriously your taking it, but i know i would be pretty gutted.
even if you DO keep the bikes secure, if they got in and found nothing, they could still nick your telly, etc, and make a lot of mess doing so.
i figure, if they go for it, they know what to exepct, they will come equipped.
either, buy a MONSTER lock and ground anchor, or hide them!
can you put all but the one you need for the next few weeks in the attic?
you could buy some noise makers too?
rape alarm style jobbys, that when they pull on a wire around the bikes, are set off, permentantly (until they run out of gas)
a few of those might scare them off?
big dog?
a sign that says "i live here" in the front window, with a picture of clifford the big red dog on it (or cerberus)
or move them! put them in a friends house for a few weeks?
good luck!