I had this same annoyance. Eventually, I ended up switching the automounting to systemd.
You have to create a file in /etc/systemd/system for the mount. Naming conventions require that to be named after the mount point with the path separator replaced by dashes. Since you have a dash in the name already, you'd have to figure out how to escape that.
In my case I added /etc/systemd/system/smb-Tomato.mount.
Description=cifs mount script
What=//<IP of server>/<path on server>
Options=guest,uid=<my UID on client>,gid=<my GID on client>,rw
Then I had to enable and start this mount:
sudo systemctl enable smb-Tomato.mount
sudo systemctl start smb-Tomato.mount
Since I wanted automount I also created a file /etc/systemd/system/smb-Tomato.automount containing:
Description=cifs automount script
and simiarly enable and start this
sudo systemctl enable smb-Tomato.automount
sudo systemctl start smb-Tomato.automount
So far I'm satisfied: the annoying broadcast message has disappeared.
After doing this I figured that just using the 'guest' mount option might have done the trick, but since I already have what I was after I did not revert to try this out.