Commercial fishing at sea should be banned IMHO. It's unsustainable, and even if quotas could make it sustainable, they are all but unenforceable.
Banning is just setting a quota of zero, which you've already agreed is effectively unenforceable. Obviously control has to be on the demand and not the supply side. Banning the sale of fish, or punitively taxing it, would be two options to make commercial fishing economically unattractive.
I'm not sure how credible that is for marine species, but I'm sure it's possible for some, and we don't have to completely cut fish from our diets.
In the meantime, I agree, we should be banning the sale/import of some species of fish.