This is one of the reasons that I prefer to stick to fantastic sciences. Normally, a character would have a power like 'Shape Water', not 'Shape Liquid', so this wouldn't come up.
But, if they actually had 'Shape Liquid', then you are screwing the player not to give them at least some control over the lava. This is particularly true if you are playing a Supers game or a 'Swords and Supers' game (like high level D&D).
Some considerations:
1) Lava is extremely viscous and heavy. The players may not be able to move very much of it compared to normal liquids, and they may not be able to move it fast.
2) Lava's liquid state requires it to remain in close proximity to an incredible amount of heat. It's reasonable to suggest that if you move it far from a heat resevoir, it solidifies and the character loses his ability to animate it.