Minus operatar should work as following:
The - operator deletes all entries that match the given name or predicate, preventing them from occurring later. [1]

When SASL mechanism selector includes - operator for some mechanism and this mechanism is added again then this mechanism can be used. It means it violates 'preventing them from occurring later'.

e.g. PLAIN mechanism can be incorrectly used when selectors like selector="-PLAIN PLAIN" or selector="-PLAIN #ALL" are used for client.

[1] https://issues.jboss.org/browse/EAP7-567?focusedCommentId=13408238&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13408238