How to remove override material to specific area (not whole grid)

[Meksikka]

I created a flat spot on my map to build a town however the override material has turned the flat spot into water/small lake. Since the dimensions of the flat spot don’t take up the whole terrain mesh square, I was wondering can I select the area of the flat spot and apply a different override material just to that area without affecting the whole square section of the mesh?

I only want it to affect the area in red.

[Meksikka]

I hope I explained this correctly. Just to clarify, I am looking to remove textures in a highly specific area without affecting an entire square of the floor mesh. Is this possible?