An easy way to end this is to prohibit the use of government funds or government labor to collect and disperse union dues.
Labor unions long have operated with a grant of monopoly powers under the National Labor Relations Act of 1935. For one thing, the NLRA gives unions the right of exclusive representation.
In theory, the National Labor Relations Board should be one of the first lines of defense against this kind of abusive behavior. It was set up 80 years ago as an independent five-member body to protect freedom of contract for individual employees, whether unionized or not. It was not set up to advance specific interests of either unions or employers. But the NLRB hasn’t followed its mission.
http://nlpc.org/stories/2015/08/27/law-review-article-blasts-nlrb-lax-enforceemnt-beck-rights