My 2nd fish ever in my tank was a Maroon Clown. A few months later, I rescued a Valentini Puffah from a LFS that is known for having crappy tanks. Soon after I introduced tha Puffah, I noticed Doink's fins had little bite marks in em sometimes. They would grow back fine, then a week later, another one. Damn Puffah was nippin' at em. It's been about 8 months since I put tha Puffah in, and I also have a tank thats twice as big as the original one they were in, and this nipping crap has seemed to stop.
Whats size is you tank? Only thing I can suggest is to often "reprimand" tha Puffah when you see him near tha Clown lol
I would point at mine, yell at him, and sometimes push him away with a net when I saw him acting up
Over time I think he decided it wasnt worth it anymore HAHA, that coupled with the move into a larger tank probably helped to stop the nipping, and lastly just time and patience. At this point, my clown will push the Puffer away using his nose. Whenever the Puffer is near Doink's "fort", Doink will come out and press his nose against the side of tha Puffah and push him away pretty hard. Maroon Clowns are pretty tough, and after they get a certain size, will usually stop putting up with bullies
If it gets too severe though, you may have to remove tha Puff and git sumthin else if you like tha Clown.