In order for a partial block to work, you have to choose the title from an auto-suggest popup. Unfortunately, the auto-suggest popup only triggers after you type at least 2 letters. This makes it hard to block people from 1-letter pages, like A, B, or C. You can work around the bug by typing 2 letters and then hitting backspace.
This will probably require a tweak to the OOUI widget, maybe creating a new parameter for overriding the regular trigger behavior.
If you type a string like A the browser will make a request like this:
however, since there is an exact match, it shows nothing in the drop down. This is confusing because the user cannot know if they made an exact match (and can press return), or if they are still waiting on results.
Exact matching results should not be filtered out (or provide a config option in TagMultiselectWidget).