Terrain Impostors Render Textures

Hi!

I would like to ask you about some problems we are facing in the engine, regarding about memory usage. In our project, we have many scenes with some terrains placed into them. In our terrains we are painting vegetation, wich is provoking the creation of several RenderTextures for Impostors that we don't need to use. We use our own optimizied prefabs and we don't need any Billboard or Impostor RenderTexture, and we don't know how to get rid of them to make room for more memory.
Is there something we can do to delete them mantaining our vegetation?

Thank you!