Generally you will find that you can't actually just stake the bonus without staking your own money. If the bonus is cashable up-front, for wagering the casino will use your money before the bonus money. So in order to get the bonus money down on a double up, you would need to stake your entire balance (deposit + bonus).
I've never actually completed a WR (well once but then bust) at CasinoRiva so I can confirm this, I get at least one 100-130% bonus email each month. I dont always withdraw right away, perhaps playing £20-£30 of winnings on slots. I have on one or more occasion just played slots only (and bust, on one occasion forgetting the bonus is sticky, so completed the WR and then lost the double up - you learn!!), so perhaps thats why I've been able to continually forfeit bonus very regularly without them banning me.