Question, how did you implement this? Reason I'm asking, is the message that you show in the screenshot should not - in my opinion - ever be visible.
I wrote this for a client, and here's how the specification worked out...
When a guest views a topic, they can only see the poll results.
When a member with < required posts views a topic they too can only see the results
When a member with >= required posts views a topic they have not voted in, then they see the voting options, with the choice to click a "view results" link to see the current results. This is standard phpbb.
When a member with >= required posts views a topic they have voted in, they only see the poll results.
So there's no reason to show a "you need x posts" to vote if you never show them the voting options.
Of course you need to check on the vote side, to keep someone that knows the phpbb structure from passing in a vote via some background process rather than going through the forums.
Anyway, just a few thoughts I have to share.
Not intended as a criticism, just a suggestion for something to consider. If you check you'll see that it's very very easy to write this way as well.