The following are two simple Nuke despill expressions (operate them on the green channel, if you are despilling green screens).

#1
g > r ? r : g

#2
g > (r+b)/2 ? (r+b)/2 : g

The first one simply means if the G is greater than R. set the G value to the same as R.

The second one is the same idea but compares the value to the average of R and B, which I find gives me better results.