Sadly, it won't stop them. They found a niche and seen that these baby bettas will fly off the shelves and now there isn't anything anyone can do to stop them from selling outside of no one in the world buying another Petco baby betta.. but face it, not many can resist.
A petition can raise awareness, maybe work on trying to get Petco to give out literature about proper baby betta care.. people read it, they may think twice to buying one due to the added work they have, etc.
Over the years I've learned you can't always stop something when dealing with corporations and their money.. but you can teach them something, change some aspects for the better, etc.
Good luck with your petition though - those babies need as many voices as they can get in hopes to grow up healthy.