This took me a while to figure out, so I'll share it in case anyone else had difficulty with it.

To remove an already assigned material in 3ds max:

select the utilies panel

Select more

Select "UVW Remove"

from the sub panel you can select your objects and remove in either the UVWs or Materials or both (in max 2012 not sure of other versions)

Good luck, sorry if this is redundant info