GG has a great question vis-a-vis the possible gender of that platy.
You are keeping the tank at a safe temperature. (Fish are more aggressive at the top of their range.)
What is the condition of your water? Could that be tail rot? (Sorry to raise the question.) Is the smaller male actually biting the larger one or just really confused/ desparate to mate with anything resembling a guppy or livebearer?
Partial water changes (maybe 25 to 45% on as many days as you can) which improve water conditions may also blunt aggression. Ironically, feeding more, while it can dirty up the water, may calm some fish. (Just think of a big holiday dinner at a relative's place. Everyone is too full to quibble.)
If removing the platy, improving water quality (even if it is already good), and all of the rest is ineffective, you may have to remove the offensive male guppy to a breeding net. Putting him a swordtail tank may make him mend his ways - he would be among even bigger bullies. ;)
You may not want guppy fry and all, but if you purchased a couple of female guppies, I would bet that both male guppies would focus on the girls.
You probably don't want to take the smaller male back to the LFS or to euthanize him. I hope one of the above ideas or a combination of them and what others might suggest will be useful to you.
All the best!
u.s.
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