This is just one of the unfortunate flaws of the current PK system.. people are able to PJ you just about whenever they want to.
The only thing you can really do is start eating right away when you get a kill, to get yourself back up to full health, and throw on your protect prayers right away. Either protect/deflect melee to avoid getting clawed, or protect/deflect magic to avoid getting bumfucked with Korasi.
EDIT: Try and keep an eye on the people around you while you are fighting. For example, if you see a guy whos within your combat range just watching, and hes holding Korasi in his hand, then it might be a good idea to slap on protect magic as soon as the fight ends. Always eat up to full health too, thats what I do.