Stop materials from blending into the materials in neighboring landscape sections.

While developing a low-poly game, I faced challenges making the low-poly look in a ue4 landscape. Here is my current Landscape Material:

I am using the DDX and DDY cross to create the polygon looking landscape. However, when I try to paint in different materials to create rocks and mountains, the materials that are painted in one section blend into another section, defeating the whole low-poly look.

Here is an example:

Is there any way to make sure each material stays in its own square that I paint it into?

Thanks so much!