Rhythm notation remove stems?

make a range selection (click 1st note/rest, shift+click last), then right click one stem, select, all similar in selection, v
or
make a range selection (click 1st note/rest, shift+click last), then right click one note, select, all similar in selection, stemless (in inspector)

But see also https://musescore.org/en/handbook/tools#fill-with-slashes

Assuming you want slash notation, then Edit / Tools / Fill With Slashes is what you should be using - much faster & easier than adding notes then making them stemless. Also has the advantage of not playing back and not transposing.

If you have some other purpose in mind, maybe you could explain it in more detail. I can't really think of what stemless notes might indicate other than as slash notation, but since you didn't mention slashes, and specifically mentioend rhythms, it seems maybe you are talking about something else. Normally, though, if you want to notate rhythms, you *need* stems, otherwise the only rhythms you can successfully notater are half notes and quarter notes.