the question is: is the value of 0 legitimate, or is the field NULL and the report converting it?
Either way, I think that all you need to is to modify the formula like:
if {SP_JD_SALESMENWISEAGENTBALANCE_RPT_RETURNS;1.RouteName}={?ROUTE NAME} and
{SP_JD_SALESMENWISEAGENTBALANCE_RPT_RETURNS;1.DocDate}={?Date} then
if {SP_JD_SALESMENWISEAGENTBALANCE_RPT_RETURNS;1.DTM200MLD} <> 0 then
{SP_JD_SALESMENWISEAGENTBALANCE_RPT_RETURNS;1.DTM200MLD}
else
""
HTH