Easily you can change the User ID of the user you want to hide.
The system tray only shows system users which are having USer ID =>1000
So to hide a user just give him a UID < 1000
You can change the UID using the command
sudo usermod -u UID username
Be carfull the numbers from 0 to 99 is reserved for system accounts, so don't use them. Moreover you should be card in choosing the new UID not to be identical with some service users in your system.
The command:
awk -F':' '{ print $3}' /etc/passwd
will list all UID used in your system So you can choose the new UID without conflicting other user.
you can sort the list in increasing numeric to be easy to notice the UID numbers
awk -F':' '{ print $3}' /etc/passwd | sort -n