The NBA has control over who the league does business with and who their teams do business with.
They absolutely could and should ban any company involved in the actions from ever working with the league or their teams for some number of years or forever.
They also could and should punish Leonard - $700K is a joke, one of the no-show deals he had with one of the companies alone was for $28M. How much $ did he pocket before this came to light and was stopped? It was almost certainly a hell of a lot more than 700K.
If Leonard was not actively pushing for and demanding extra pay outside of cap constraints and, instead, refused to cooperate with this activity, nothing would have happened. The companies, agent and team would have never set up the scheme to begin with.
If there truly are no rules against players engaging in (and, in fact, demanding and orchestrating through family members and his agent) cap circumvention activities, that would seem to be an absolutely cavernous hole in the rule book that would need to be filled for the good of the game as a result of this disgraceful behavior.
They also could and should veto the trade the Clippers made with the Raptors or, if they want to move forward, put iron clad restrictions in place so the Raptors & Leonard will not engage in similar behavior with very clear punishments defined for all parties. Unless Toronto & Leonard agree in writing, the trade should be cancelled. Leonard should get zero benefit of the doubt after his behavior.
The agent should be banned from working with NBA players for a number of years or forever.
The commissioner should formally define in a letter to every NBA team the consequences of similar behavior and the zero tolerance for undermining the integrity of the league and cheating the game. And if, again, those rules do not exist currently, they should be voted on and approved as soon as possible to protect the league. If the players union fights it and will not accept such rules, then a collective bargaining resolution will need to be found, but I would hope that players understand how corrupt this behavior is and that it cannot be allowed.