Gideon's link is irrelevant to the issue you have. You might prefer to read this other link (http://www.question2answer.org/qa/11611). In there Gideon mentions the core accepts a new module type that allows plugins to hook in the question creation process.
Interestingly, Scotts' plugin has the feature you're looking for right there as he implemented in his plugin the filter Gideon mentioned in the link above. I thought it only aimed at handling tag duplication but it also limits tags' creation. This is the link to the plugin: https://github.com/svivian/q2a-tagging-tools
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