I've caught A LOT of fish on a hook and line - and not just in aquariums. (That's cheating!!!

)
Significant damage to the fish is possible - so is significant damage to your self if you aren't careful - but very, very unlikely IME.
Significant damage (even death) from stress of being chased is guaranteed - even for the fish that aren't being chased and the owner too.
If someone already had a trap I'd say use it since it's the least stressful (as long as the fish doesn't have to spend any time in the trap! Don't use a trap when you aren't there to tend to it!), but IME fish get wise to them very quickly. I wouldn't spend much time waiting (<48 hrs) for it to work. I would not spend money on a trap in most cases. Depending on the fish, there are DIY traps that could be worth trying, within the aforementioned limits.
Of all the posts so far, draining the tank to facilitate netting or catching with a hook are my favorites.
Edit: Somehow I missed post
#2!
The mirror trick. (Place a mirror next to the tank somewhere - hopefully the Powder thinks it's another of his kind and starts ignoring everyone else.) Not quite a guarantee, but easy and low-stress to try out! I wouldn't spend a whole lot of time (<72 hours) waiting for this one either, however. Either it'll work or it won't.
Unless you are trying to save another tankmate from the offender, try whatever is easiest for you first. If it's a matter of saving another fish, I'd find my closest fly-fishing supplier a.s.a.p.
-Matt