Unfortunately I think you are screwed.
The problem is that you played on after they added the bonus to your account - as soon as you bet 1c of the bonus you are deemed to have accepted it and the terms which go with it.
What you should have done of course, as soon as you saw that pop-up you should have stopped playing and contacted CS to ask them to remove the bonus.
Regarding the "law"; basically there isn't any when it comes to online casinos. There is some supposed "regulation" by the country/body who licence the casinos, but in the majority of cases the licencing authority doesn't give a toss, to be brutally honest.
In any event, even if the casino was licenced by a responsible authority, I think in your particular case they would still say the casino is in the right for the reasons I gave above.
Sorry you have learnt one of the "Online Casino Lessons" the hard way - at least now you wont make that mistake again!
I'm afraid your only option is to complete the WR and hope you don't lose to much in the process.
You never know - you might even increase your balance!