Spell it out.
Best to spell it out as well as use a phrase such as "wagers that cover all outcomes etc". Any covering phrase would have the "abusers" looking for the loophole, and spelling out the bets will not help the abusers as they know them anyway, and genuine players will see plainly what is meant by "abusive wagering", rather than them believing it covers normal clever wagering, such as small stakes and strategy.
Roulette is the most complicated game to cater for, as there are many ways to hedge all outcomes, however, the house edge never falls much below 2% (French rules), so it would be enough to ensure the WR are enough to prevent their being any overall +EV in abusive wagering.
The other type of wagering that has recently become "abusive" is making large high risk bets with the whole balance on a near 50/50 outcome, and then completing WR more conservatively if this wager is won.
One idea I have seen is a flexible WR term, such as "WR is nn x D+B OR 100x the amount of your maximum bet". This ensures that the leverage gained by making one big winning bet to start with is lost, as it causes the second term to dominate the WR of the resulting balance.
You must remember that casinos are in business to make a profit, and players are out to WIN!!
Another idea is to run a different promotion each month, thus making it harder for the websites to keep a blow by blow guide to beating the bonus up to date. Software choice is also a factor, it should make it easy for players to follow the progress of their progress towards completing a promotion, and prevent players from placing invalid wagers from the outset.
The aim should be to never have to be in the position of confiscating a player's winnings, with a worst case scenario of confiscating a bonus if it was improperly applied for and letting the player keep their deposit and winnings (although the option to close the account, or ban more bonuses, afterwards is always there).