are you using single hex or double hex socket?

if you are using double socket that's your first problem.

if it hasn't rounded off yet try using 9/16 mm single hex socket tap it on with hammer and use a breaker bar and undo it.

if it hasn't rounded off yet try using 14 mm single hex socket tap it on with hammer and use a breaker bar and undo it.

if it hasn't rounded off yet try using 1/2 mm single hex socket tap it on with hammer and use a breaker bar and undo it. ( if you are using 1/2 you have gone way beyond it and would probably best using a chisel and hammer ).

also if you have a cheap set of spanners you are best off trying to use them as the cheaper sets a usually out of accuracy which will usually fit over well! hope that helps!