My top five no no's would be;
1. Dragonets, regardless of species for obvious reasons.
2. Anthias, regardless of species. Beginners do not understand the feeding requirements and water conditions to keep these in top form.
3. Those tangs that are considered expert level, ie. Achilles, Cheveron, Clown, Sohol and the remainder of Surgeonfish genus also to include the Moorish Idol. Leaving bristletooth tangs and others available.
4. Butteryfish in general, these fish require proper acclimation and tank mates. Many are obligatory coral feeders, the common beginner has no clue.
5. Angelfish, for the most part. Here again too many of these are difficult to acclimate, tank mates stress them leading to disease and death, many are considered expert level.
Not to add a sixth; but the fish that should not be harvested in the first place. Those that grow to sizes that are not feasible to keep in a normal home aquarium. Batfish, lookdowns, grouper, most eels, mono's, jack's, puffers, sharks and rays. I find many of these species quite endearing and would love to have them in my home, but realize that I cannot provide the space required for them.