The National Labor Relations Board has ruled it is illegal to force union-backing employees to attend meetings held by their companies that are designed to undermine support and argue against the labor groups.

Amazon also removed union-related posts on an internet message board and threatened to discipline a JFK8 employee if she re-posted them, the board found. Workers at the warehouse ultimately voted to join the Amazon Labor Union, making it the first and so far only Amazon facility to unionize.

Mandatory “captive audience” meetings in which companies argue against unionization are illegal, the National Labor Relations Board ruled in a case involving Amazon.com Inc., prohibiting one of employers’ most potent weapons against labor organizing campaigns.