I am sorry if you don't like it, but this problem has been around for at least 40 years that I personally have seen, even back on the Viscounts in the 1960s the Hosties insisted on doing it no matter how many times they were told not to.
IF what you said is true, then your Employer should provide some sort of container for these grounds to be kept in for the duration of the flight, so you don't "accidentally" put these grounds down the drains.