If returning whatever value is before the decimal is your only goal then you can just do some string manipulation and cast it as a number. In the following example, imagine that the [x] measure is your value:

MEMBER [xFormat]

AS

StrToValue(LEFT([X], INSTR(CSTR([X]), ".")-1))

There are floor functions available in MDX (via vba) but they do not do as you describe as far as I know.