If it is the part i think your talking about, you have a plastic/rubber slide on the swing arm and, depending on your bike, a rubber/plastic roller on top. I usually use petrol or turps on a rag to clean off all the old chain lube and grime.
Thats ok too, but I was talking about the I think it's called the chain block? The rubber piece that fits inside the metal thing? It's located underneath the sprocket, is petrol petroleum? And what's turps? Thanks!
petrol is petroleum, turps I'm guessing is turpentine or mineral spirits. Get a skinny nylon brush and some simple green or other degreaser. Spray the block and let it soak, then just brush off the crud.