In my opinion (and I say this as a commish myself) it is not the commissioner's job to babysit players from making bad trades. Now, if Team B is long out of the playoffs and Team A is still competetive, and you have reason to suspect intentional collusion between the two teams-then maybe. But otherwise no.
Also, there is no saying for sure that that trade doesn't end up working in the favor of Team B if Isaiah continues to be hot and Amare finally puts it together, and Pierce fades.
So I say absolutely not. It's not your place to veto that unless you have good reason to suspect collusion.
You should never be a commish then, "you don't take the responsibility seriously", you sound like a ho hum type of commish. There are other people in the league that expect a commish to be more than that, you do have a responsibility to them too.