Hi,
I suppose that by "MyParcel locator plugin", you're actually talking about the "MyParcel pickup location selector plugin".
In the settings of the shipping method(s) you can create via the "new" button of the System>Shipping methods menu, you need to select the carrier to be used and the country supported.
So if MyParcel only supports Postnl locations for Belgium and you get BPost locations on the frontend for users with an address in Belgium, this means that you've selected "BPost" as a carrier and "Belgium" for the country in your shipping method. Can you check this ?
And in that case, you can just change that carrier setting to Postnl and it should prevent the issue.