There must be a quick and fast solution, given Lilypond's awesome power, but I just can't seem to find it. The closest I got was to override Stem.thickness, but apparently it cannot be set to 0---a faint stem is printed nonetheless.
I knew it had to be super-simple! And I got three solutions in less than 8 hours:
a. \hide Stem
b. \hide Staff.Stem
c. stemOff = {\omit Staff.Stem \omit Staff.Beam \omit Staff.Flag}
\stemOff
Kudos to Fernando Gil for his most complete solution (c), even though the shorter (a) and (b) worked as well in my easy use case.
Thanks everyone!