Hmmm...I have no idea...But I strongly feels that the long term solution is to have a bigger tank

And if even after that, you're still having the same problem...well, maybe, just maybe the guppy is a delinquent. And personally I would placed such fish into solitary confinement...hehehehehe...for a while at least

Hopefully, by the time he had done his time and placed back into the tank with the other fish, there would already be a shift of hierarchy and he may find that he is no longer the king of the tank.