I submitted my EE profile on April 12th, received Ontario provincial nomination the same day. Before I could apply for nomination, I got my ITA (April 13th). I haven't yet applied for provincial nomination because I think I don't need the extra 600 points anymore. Is there any other benefit of applying for provincial nomination if one has already received an ITA?
If you already got the ITA then you are good to go, you can ignore the nomination.
On the other hand, you may be confusing nomination with Expression of Interest. Ontario does not nominate directly from the pool (unless you applied previously through a different non-EE stream)