This has happened to me a lot - generally I’ve just cursed and strained away with pliers or screwdrivers (wedged in between pads to act as a lever - e.g. lever the piston back in) if you perservere it will move - then when you’ve got a bit more of a gap stick the calliper on the disc and bang it on with a rubber mallet.
Bandit rear callipers have a habit of seizing up because they are underslung and exposed to the worst the road can throw at them - this is why they need regular cleaning and greasing - I expect this is your first go at them hence the trouble - when you’ve sorted it at least there will be the consolation that it will be easier next time.
Fingers crossed someone will come on with a more clever solution than the combination of bloody mindedness and brute force I have outlined above.
If the fluid has been topped up while the pads were worn you could find that as you try to push the pistons in , the master cylinder fills up to capacity preventing any further movement.
As choprocker said ,bleed a bit out and personally i allways bleed the brakes every time i do the pads but you at least need to check the level
I’m dreading doing the rear pads on my Bandit - calliper is a really terrible design and the pins have totally seized. Only way to get 'em out would be to split and rebuild the calliper, or drill two small holes in the calliper where the back of the pins sit, and poke them out :doze: