The creator will always have access to PNRs they created. Makes sense, since they will ultimately bear responsibility for that PNR. (I haven't heard of any case of travel agent being denied access to their own PNRs)

The operating airline of each segment will have access to the PNR, but they will not be able to see the remarks and certain other fields of other airlines or the travel agent (unless placed in an area where everyone can see it). I believe they will also not be able to change segments other than the airline's own, in normal circumstances.

Other travel agents will not be able to open the PNR. Airlines that will not operate any of the segments will not be able to open the PNR.

In response to your question, I don't think you will be able to block the travel agent out of your PNR if they created it.