For removing grease,I've had good luck using GoJo hand cleaner(also great for cleaning your hands).

For preventing it in the first place,I've had several bikes with the guards GravelMN mentioned,and they work really well. There are full coverage guards like the SKS Chainboard,but they can be a PITA to mount,and can be rattlely.

In shorts weather,I'm just careful not to brush the chain. In long pants weather,I use one of these;apparently,they also make an even longer one,but I've had good luck with the standard.