Try penetrating oil on the fixings first - be generous, and leave it longer than you thought, overnight for example, if that fails then its resorting to heat as Fanatic says and/or hacksaw or grinder if you can get access.
Top tip if you have a decent spanner or socket which fits well - lengthen the "arm" and increase the leverage with a short length of scaffold pole ( one end on the nut, other end inside the scaffold pole) - works a treat to crack off fixings which have had rust loosened with penetrating oil.
Watch your knuckles though - you might be off piano practice for a week................