Group on OrderUser.
Create a formula to "flag" orders pre this month:
if datediff("M",{table.datefield},currentdate)>0 then 1 else 0
Create a Summary as a SUM on this formula field at GF1 (per OrderUSer)
Suppress anything where the SUM>0 (indicating this orderuser had an order greater than this month)
You can change your "flag" formula to day differential if you want it to be something other than just in the actual month, like 31 days.