If some one is that desperate to steel, nothing is going to stop them, but a roof rack will be bolted down, and ladders clamps can be padlocked
Roof bars can be popped off with screw drivers, I suppose it's the same we're ever ya go, but I don't trust the bars.
I had a few Clio vans a while back and was using roof bars, I drilled holes through clamps that go onto van and put nut & bolt through all 4.
2 reasons for this, 1 yes to stop thieves, and the other was I seen a set of roof bars come off a van on m6 just between junc 21-21a, luckily and it was by luck that it never hit nothing, he was bout 400m ahead of me, the back ones came off and just ripped the front ones off with them, by the time he had started to brake his ladders was behind him sliding onto hard shoulder,