If you really want to remove all of them from a map, you can do it by making it into a custom scenario. Load up worldbuilder after you create the map (control + W I think). Save the map as something (e.g., "MapToEdit.CivBeyondSwordWBSave"). Close the game. Navigate to the folder that was saved in (by default, <My documents>\My Games\Beyond the Sword\Saves\WorldBuilder I think). Open the file with any text editor (word or notepad work fine). Use find-and-replace to replace all "TERRAIN_TUNDRA", "TERRAIN_SNOW", and/or "TERRAIN_DESERT" with some other form of terrain. Save it, then load that up as a custom scenario.
Alternatively, you could copy-paste your CIV4TerrainInfos.xml file (from Beyond the Sword\Assets\XML\terrain) into your Custom Assets\XML\terrain folder (which is in the same general area as your worldbuilder save would be), then use some judicious copy-pasting to "over-write" Tundra terrain with any other terrain of your preference. To give a simple example, you could take everything from "<Civilopedia>TXT_KEY_TERRAIN_GRASS_PEDIA</Civilopedia>" to "<bGraphicalOnly>0</bGraphicalOnly>", copy it, then paste that over the same portion for TERRAIN_TUNDRA. Then you need to also create a custom assets copy of CIV4ImprovementInfos.xml - everywhere you see "<TerrainMakesValid>" followed by TERRAIN_GRASS, you're going to need to add another entry making it valid for TERRAIN_TUNDRA. That should make all "tundra" look and act the exact same as grass, in every game. If you decide you don't like it, just delete those two files from your custom assets folder.
As other people have been mentioning, you'll be in for some weird games if you do either of those.