That's a tricky one. I like using a probe to be able to monitor my basking temps too (I like the fact that it stores min/max temps, so I can gauge what is happening throughout the day, even when I am not there). But I have had chams try to climb along wires and misting tubes in the cage from time to time.
What I tried to do to prevent this was run the probe wire tight along the underside of the branch the cham basks on all the way to the side of the cage (i.e. make sure the branch goes all the way to the side of the cage), and then to run the wire down/up the side of the cage flush with the wall. This ensures that there is no 'perch'/'highwire' created by the probe cable for the chameleon to walk along.
Another thing that might solve your particular problem is that as the chameleon grows older, he will become heavier, and the probe wire will no longer be able to support his weight. He will stop climbing on it when that happens...
In the meantime I would be less worried about him falling (because they are built to avoid falling, and also built to save themselves if they do fall), and more worried about what his claws are doing to the probe wire...
Like I said, try neaten everything (using cable-ties) so that everything is flush, and he won't have anything dodgy to climb on.
