Sherman Act extension, anyone?
This could be reasonably construed by any competent AG (if only we had one) as restraint of trade...
Further, per 15 USC § 3 any such act committed in the District of Columbia is a violation regardless of whether it was otherwise "interstate commerce."
Group Boycotts
Any company may, on its own, refuse to do business with another firm, but an agreement among competitors not to do business with targeted individuals or businesses may be an illegal boycott, especially if the group of competitors working together has market power.