Note stems

If you right click a measure and select measure properties there is a check box for stemless. Otherwise you can select a section of notes in the inspector (press the note button) then mark with no stem for all of those notes.

Any reason you're attempting to do this manually and not using Tools → Fill with slashes?
See also https://musescore.org/en/handbook/tools#fill-with-slashes

Definitely, "Edit / Tools / Fill with Slashes" (and/or its partner "Toggle Rhythmic Slash Notation") is what you want. Manually adding slashes and removing stems doesn't really do the whole job - the notes will still play back, plus they will still transpose, plus they will export to MusicXML as normal notes, etc. If the goal is to create slash notation, it is important to use the facility provide for the purpose rather than trying to fake it manually.