HI Vishal,
Did you Try this
IF(DayNumberofMonth(CurrentDate())=31 Then Month(Relativedate(CurrentDate();-31 Else Month(Relativedate(CurrentDate();-30))
Let me now if doesn't Work
Or else you can Try as You have Report Month you can Add block Level Filter or Query Filter and Keep NOT INLIST ( Month NUmber what you want to Ignore)
Hope it Make Sense
Thanks,
Varun