You can use the CLEAR block to clear all pen and stamp marks, but there is no block to selectively erase stamps or pen marks (though it *has* been suggested to the scratch team---it would be a simple matter of allowing "erase" as a pen color).
What some people do to fake it is to have a sprite that looks just like the background and stamp it to "erase" previously stamped things. This only works with simple backgrounds or where what you need to stamp/erase is always in the same place.
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