It seems obvious that Mechs should not be able to enter the spaces of or cross the Lava Wall. However, the current rules don't mention what happens when they try to do so!
It seems clear any Mech should at the very least stop its movement when it reaches the Lava Wall, but thematically, you would think it would take damage if it had excess movement, as if the Lava Wall was like a Spiked Wall.
So what is the method we should go with here?
Have you taken a look at the Watch it Played FAQ uploaded here on BGG? I'd scan through it myself for you, but I don't want to spoil anything.