Take off the plaster. Have the plaster taken off. Technically, we don't do this stuff ourselves. We have doctors do it for us. His arm was put in plaster. He had his arm put in plaster. The plaster was taken off. He had the plaster taken off (his arm).

Normally we use 'remove' or 'take off' the plaster.' The opposite of 'put on' is 'take off'. 'They removed/took off his plaster at the hospital yesterday'