Dear Jeremiah,
I just want to say that it's all about the design of Release strategy which works accordingly, the way you set the Release strategy.Please be noted that if the approver is only one in your company for all the plants, than definitely you have to assigned only one release code in authorization object in approver user. Normally what companies do, they usually make Release code according to Release authorities/approvers, if there are multiple persons/authorities who are responsible for their respective plants than you have to make separate code for all such persons and separate release strategies as well, and assigned release code accordingly in their user, by doing this person can't releases other plant's PR.
Now coming to your question, let's assume that person is one, and you have release code which is also one than definitely he can release PR, other than his own plant.
However you can run SU24 and check the authorization object which are available for Tecode ME54N
M_BANF_EKG: Purchasing Group
M_BANF_FRG: Release Code
M_BANF_WRK: Plant.
You can ask your basis guy to restrict user by these authorization object as well.
Best Regards
Mohsin Abbasi