Hello Annali,
This code should hide the entire menu from view:
.jsenabled .usermenu .moodle-actionmenu.show .menu {display: none;}
You'd need to provide the user/login block so that users could logout properly.
Something else you can try instead of the code above is this. This will try to target just the preference item in the list.
.moodle-actionmenu[data-enhanced].show .menu li:nth-child(3) {display: none;}
None of this is truly tested so you'd have to run down different screen sizes and devices to ensure it stays hidden.