I don't have a solution, but I certainly feel for your plight. I bought a =A350 9" grinder from Screwfix and the nut locked solid on the first disc, I bent the spanner trying to get it off. So I took it back and got a replacement which got through 2 discs before the same thing happened. So that went back too and this time I got a refund.
I then got a =A330 9" grinder from Argos and went through one disc, then the second one locked solid. Got a replacement and the disc stripped off the grinder leaving a permanently locked, unmovable nut. So that's going back for a refund too.
I don't know if it's just that they are cheap grinders or if it's the locking mechanism, which seems to be the same on all of them, that is useless.