I can´t see how... levelblocks sadly don´t offer the "nofilter"-flag.
I suspect that it could be an "D3d_...."-flag.
I tried it with "d3d_mipmapping = 0" and the distant textures looked a bit sharper, but the textures themselves are still softened.
There must be a possibility to turn it off, I mean it´s a absolute basic rendereffect since GL-Quake.