Find out what type of glue is on your enamel tub, if possible. A simple, white glue can be easily removed by peeling it off or dissolving with water. However, removing a more complex glue, such as Super Glue, will require a heavier solvent.
Try to peel the glue off the enamel tub using your fingernails or a butter knife. Remove as much as possible by hand.
Soak the remaining glue with an adhesive remover such as "Shout." Allow the glue to soak for about 30 minutes and then wipe off with a dry, clean cloth. Repeat as necessary until the glue is loosened and removed.
Wash the tub as you normally would, once the adhesive is gone, using a liquid detergent or all-purpose tub cleaner with water and a sponge.