Add the following to the top of proftpd.conf...

<IfModule mod_ratio.c>
Ratios off
SaveRatios off
RatioFile "/restricted/proftpd_ratios"
RatioTempFile "/restricted/proftpd_ratios_temp"
CwdRatioMsg "Please upload first!"
FileRatioErrMsg "FileRatio limit exceeded, upload something first..."
ByteRatioErrMsg "ByteRatio limit exceeded, upload something first..."
LeechRatioMsg "Your ratio is unlimited."
</IfModule>