Usually people who are nominated as approver are intelligent enough to select and release only requisitions in their responsibility. It is even possible to enter the plant in the selection screen of ME55 to get only the PR for which he is responsible, minimal education could help too.
People who have authority to work in different plants have to be careful anyway to not mix the plants when they enter requisitions or anything else.
It is a long known fact that authorizations accumulate per object. Talk to your security team for details. Such facts have to be considered when you setup release strategies, even it leads to the effect that you you have to setup 300 release codes.
And if you don't want to do that then you have to go the ABAP way to program own checks.