Hi John,
Sounds like you are talking about some kind of limpet perhaps?  Limpets are a
problem because of their ability to clamp down, forming a watertight seal
between the edge of their shell and the substrate.  This prevents you from
getting a knife or other tool under the shell without chipping it.
Unfortunately it also prevents any chemical you sprinkle or pour on them from
getting inside the shell.  I have had some success with limpets (and chitons)
by slipping a fine hypodermic needle (27 guage) under the edge of the shell
and injecting alcohol.  They fall off the rock within a few seconds.
Probably a strong salt solution would work too.  However, I realize most
folks may not have access to this type of equipment.  I have often thought
that keyhole limpets and abalone should be easy to remove by injecting an
anesthetic through the natural openings of the shell.  However, since I don't
have either family locally, I have not had the opportunity to try it.
Regards,
Paul M.
Rhode Island