I've never really understood why people would tear down someone who writes a Mary Sue. Often these writers are only children.
Don't let them win.
If you see this happening, speak up! Remind people that everyone deserves to be treated politely, no matter how well they write or how long they've been writing.
If this happens to you, block them. Bullying is not
constructive criticism, and you don't deserve it. Above all, don't let them stop you from writing.