The President can order the stopping of sales of military grade weapons to local police departments, if they don't pass muster. It has been pointed out that the police departments often get their weapons from the U.S. military.
That's constitutional and within his authority as Commander in Chief.
If Congress has ordered the sales, then it's not. Maybe marginally it would be if he issued a finding that the US military still needed the equipment Congress ordered him to disperse to the States, but I can't see Obama saying we need more military.