The macro delete-advice (or the function remove-advice) may be used to remove a named piece of advice. Since several pieces of advice may be attached to a single functional definition, the name must be supplied to indicate which one is to be removed.
CL-USER 40 > (delete-advice capi:prompt-for-file pff-1)
NIL
CL-USER 41 > (delete-advice capi:prompt-for-file pff-2)
NIL
LispWorks User Guide and Reference Manual - 20 Sep 2017