ahh..ok
I assume you mean one of the round (or square) frag plugs with the 'stem' on the bottom? If so, I usually snap that piece off with bone cutters (or you can use a hacksaw as long as you don't keep it out of the water for more than a minute or two, just score it and use a pair of pliers to snap it off ), and then glue it onto your rock.
If you can easily get just the Duncan off the plug, you can also simply glue it onto your aquascaping as well.
I think frag plugs stink and try to get them down to as small as I can, if I can't get them completely off. You don't want to lose the coral though so it up to you on how comfortable you are hacking away and gluing animals to rocks; the plug doesn't hurt anything.