Every time it jumps up onto the keyboard, hold it tight and push your finger against it's head, just above and inbetween the eyes. Wiggle it a little and see if the cat doesn't like it. If they don't, keep doing it until they back their head away; then pick them up, put them on the floor, rub them on that spot again and usher them out of the room. Also combine a tap on that same spot with a wiggle of your finger (like 'no no').
If you do it everytime it happens, and say 'no' in a very harsh and abrupt tone, it should help. Basically, just do something to the cat that it doesn't like whenever it jumps up onto the keyboard, and it should stop doing it.
Henrah
I've done this to my two cats, and now I can stop them from attacking each other and eating out of each other's bowl through a simple voice command

It's all in the tone of your voice.