Call me old-fashion, but you could always sew his mouth shut.
Literally OR metaphorically.
You could use a doll or even a photo, to sew their mouth shut. Perhaps saying a spell while you're doing it. Something about 'silencing a slanderous tongue' or 'muting his malice lips.' I always enjoy a bit of novelty paired with some poetry.
I've created a spell based on your ideas, if that's alright.
Because of the three-fold rule I do my best to stray away from spells that could potentially cause harm in myself and others, and do not wish to bend free will to suit my needs. So I always try to make my intentions clear so nothing bad happens.
With your idea I've thought to combine the doll and the picture, if you don't mind. I just thought it was a very good idea is all
Now thinking of this person may bring up feelings of anger or resentment, so I would say your best bet to avoid bringing the other harm would be to meditate for a little while, and try to think of their positive qualities, and realize that everyone is human, everyone makes mistakes, and this happens to be theirs. Maybe even think of some things you've done to anger another person, and the guilt you felt, but you were forgiven, right? So try to let go of your anger and only wish them the best, to avoid unintentionally bringing this person harm. Keep in mind only doing this spell because you feel this person won't bring their self to care about the harm they're bringing you, and think of this spell as a sort of nudge in the right direction, only wishing for respect.
Get a picture of the person you wish to stop spreading rumors about you, and cut out the head and attach it to the doll's face. I'm a very visual person, so this combination would help me more than simply using a doll or just a picture.
Grab some thread and a needle - I would use black thread, to symbolize the banishing of ill-speak, as you will use the thread to sew the mouth on the picture shut.
This can be all you need, or you can add more. I haven't done a spell like this, but if i were to do it sometime in the future, I would use some candles.
Black to repel negativity.
Brown for friendship.
So to begin, take your doll with the picture of the person's face whom you wish to cast this spell on. Get your black thread and needle ready to use.
Light your black candle as you visualize the negativity leaving the person spreading the gossip.
Light your brown candle as you visualize you and this person sharing mutual respect.
As you chant this spell i've written, begin to tightly sew the mouth of the person in the picture. You whose mouth always fails to cease
Saying such things to disrupt my peace
I bid you from this hate you so desperately cling
from the anger and resentment you continuously bring
Your lies and hate end right here
to start anew with love so pure
Now bid farewell to such ill-will shown
for it shall be muffled by the thread i've sewn
and with my intentions only innocent and sweet
As i will, so mote it be.
I hope this helped (: