Ok, now I am going to probably catch some flak for this, but the easiest way (not the nicest) to remove a damsel from a reef tank is to....fish for it. Yep, I know... I use small trout hooks, bait it with whatever, and catch the little bugger...transfer him gently into another tank or recepticle and remove the hook (to ease this, I file off the barb on the hook) I haven't had one die on me yet (knock on wood)...
Another option is to use a glass jar to scoop up the damsel, but this can take forever...and yes, I do feel a little silly baiting a hook and lowering into a tank. But it does work.
I've got two damsels cycling my tank right now. I've got another one they can go into if need be when the octo arrives, however I might just see how it goes. These two fish are the dumbest organisms I have ever come across: they will be swimming less than an inch away from a chunk of food and not touch it. On occasion they will run away from food. I've had them for about a month and I've only ever seen one of them eat.