Can i defy a rebuttal that attempts to steal my discipline

Yes. Defy can counter Rebuttal before it resolves, so your Discipline stays yours.

Rebuttal is a spell with Energy 1 and Power 1, so it's a legal target for Defy (≤4 energy, ≤1 power). Since both are [Reaction], you respond to Rebuttal on the chain with your own Defy; the Defy resolves first (LIFO), countering Rebuttal (Rule 425.1.a). The control-change never happens, and your Discipline then resolves normally. This only works while Rebuttal is still on the chain — once it has already resolved and taken control of Discipline, it's too late to Defy it.