seperating guppies

You can buy plastic mesh canvas (craft section in Wal-Mart used for needlepoint). Cut it to fit front to back & height of your tank and divide a section of your tank by pushing the bottom into the gravel. Perhaps support it on either side with a ornament or small rock. The tiny holes in the canvas allow water flow-through for your heater and filter. This is the same effect as a commercially-bought aquarium divider but much cheaper. If you want it in place more permanently, you can buy report covers (the kind with the plastic V-shaped binder you slip the cover into) and slip the ends of the canvas into those so they rest flat against the glass.

you could also buy a cheap 4 dollar breeding net at petsmart or somewhere like that. It would keep it seperated from the rest of the fish until you figure out what to do.

I dont really have any new advice on separating because you already got that advice, but you could go and buy a couple more guppies to hopefully cause his aggression to be divided up and not harrass one fish to the point of sickness.