I thought that Fotolia only offered that option for exclusive images, but I'm not aware of anyone else who offers an opt out. Even at Getty, they modified their contract several years ago to give them the option of moving RM images to RF and even to subscription if they wanted to. No opt out from that.
You can license direct (lots of control, no built in buyer audience); you can sell only JPEGs of your illustrations via microstock, or select only some of your illustrations that you're OK with selling in higher volume at lower prices.
Once upon a time, iStock was a great sales outlet for very complex illustrations - because they had a pricing scheme that based things on the complexity of the work. Simple things were cheap, but the really elaborate works (which would sell well) commanded much higher prices. Those days are done though, plus they have subscriptions with no opt out.