If you're certain you're using the latest ATI drivers, there are 2 main things you can do:
1. Paste the following into your Blockland console and press Enter.
Pref::OpenGL::noVertexBufferObjects = true;
2. Wait for v13, as it addresses some of the geometry corruption issues.
ATI drivers, especially those for older cards, are known for having less than solid OpenGL implementations. Blockland is an OpenGL (versus Direct3D) game, so many people with ATI cards report graphical problems.