if we were talking about MojoWorld, it'd be simple and obvious -- use a "Min" node. the procedural terrain function plugs into the Min 1 value, and the clip height constant is specified as the Min 2 value. whereever the procedural terrain height is less than the clip height, you get procedural terrain, and whereever the procedural terrain height is more than the clip height, you get perfectly (and I mean mind-numbingly boring flat) clipped plateaus.
I'm *sure* the same functionality must exist in T2 - it's simple mathematics. however, I haven't yet gotten building terrains with functions very far along yet. maybe the conditional scalar would work for this, but I haven't been able to make that do anything in my few guesses at how to use it.