... if able? So they don't do anything if they don't have enough mana? ... and they decide how they pay for that mana?

I mean, this isn't trying to be Rethink is it?

You can't be forced to activate mana abilities, so all this does it empty their (presumably already empty) mana pool. What you want hasn't worked since they got rid of mana burn

­Drain Power shows us how to do this correctly. But instead, I'd suggest being a bit more fluid with what's going on and try something like this:

"...Its controller taps X untapped lands, where X is the spell's converted mana cost. Then that player loses all unspent mana."

I thought drain power showed us "You know what? This feels like it ought to be posisble, but it kinda doesn't work properly. Let's not print that again."

Which is a shame; because, well, it should be something you can do :)

I think "The lose the next X mana they gain this turn" might work?

Ooh. That's clever Vitenka. I not sure what happens when a player gains and they only lose two mana. But I want to believe it works just fine thank you very much.