It's pretty common, total nightmare! What it is the bolt corrodes into the metal sleeve inside the bush. If you can get into it cut the bolt either side of the bush. Don't forget that the inner lower arm bolt that attaches to the subframe adjusts the toe/camber so will need setting up after.
Send a message to peevee on here, he has recently refurbished his rear subframe so should be able to give you some pointers.