Ah. Welcome to my world. If they're alloy or ali bolts then just drill them out. They're quite soft. If they're stainless, like the shimano ones then it's a lot harder and you'll probably need help. But if you can drill them out you'll need to buy a new set from a bike shop. Many bike shops don't have these, so phone around. KCNC are the easiest to get usually. Then, before you buy them, make sure they're the right length. If you have a triple crank then they need to be long enough. If you have a double, they need to be short enough to bite without any wiggle. If you, like me, keep putting single rings on and off, they'll need to be very short. When you put new ones in, use locktite or something like that, because they strip really easily, and because they're such an extreme point of force on a bike they loosen really easily too.
