Once silicone is cured (dried) its really had to remove if white spirit hasnt shifted it (together with a lot of elbow grease) you could try that 'Sticky Stuff Remover' that says it removes chewing gum n stuff.
Im sure tho that a few remaining bits wont affect the result of applying new sealant.
Tip: almost fill the bath with water so that the gap is at maximum before putting on the new silicone and leave the water in till its set, if you dont the first time you run the bath it might move away from the wall and take the sealant with it.