Manually changing the friendship level won't delete them from the list of friends until the relationship itself is also culled. Which happens at specific days/times. UI Cheats does remove them if you set it to 0 though, but running just the cheat code to do it won't.
If you are willing to use a mod solution, a fast solution for removing multiple friends at once is to use MCCC. With that mod you can click your sim, select "MC Command"->"Relationships"->"Delete Relationship" and then you get a sim picker where you can select every single sim that you want to remove from your Sim's friend list. It applies instantly, sets the relationships to zero, and removes them from the list of sim's relationship panel entirely.
Another mod solution is Carl's Gameplay Overhaul mod which adds a new cheat code "carl.fixfriends" and if you type "carl.fixfriends 20" in the cheat console, for example, it purges all relationships of 20 and below for the currently selected/active sim. You can use whatever number you want in place of the 20, that was just my example.