The only valid way to criticise someone's art is to make better art. You're not allowed to just say "that sucks", and you're definitely not allowed to say "that sucks and you should stop making art forever". If you want to say some artwork is bad, whether painting, drawing, writing, acting, film-making, music, whatever, then your only recourse is to make something better.
Mokalus of Borg
PS - If your retort is "but I can't", then you're making my point.
PPS - The world has enough critics and not enough artists.