Step-by-step guide
Let's say you don't care about the GRIB data values (and its bitmap) and only want to keep the meta-data, for example when preparing a sample file (Also see Can I use my own GRIB/BUFR sample files - ecCodes GRIB and BUFR FAQ)
This can be done by using the "-d" option of grib_set as well as the key "bitmapPresent" :
% grib_set -s bitmapPresent=0 -d0 in.grib out.grib
Here we set all the field values to zero (-d0) in effect making it a constant field. Also if the input had a bitmap, it will be removed.
Now check the sizes of the two files; in.grib should be much larger than out.grib.