See this thread for pictures of the "tangs"...
http://forums.pelicanparts.com/911-engine-rebuilding-forum/518885-installing-injector-sleeves.html
You did not state whether the manifold runners are on or off the engine. If off the engine, removing the plastic sleeves is as easy as using a socket extension from below to tap them up and out. Shouldn't take much force as long as the tangs are ground off. If the manifolds are still on, obviously you need to be very careful nothing falls into the cylinders when removing the sleeves.
My advice is, unless the runner-to-head gaskets have been replaced recently, remove the runners, knock out the injector sleeves, and replace the gaskets while you're at it. They're not expensive.