There's actually a Change.org petition demanding Instagram give all users the ability to disable comments "to stop bullying." I've considered finding the user's Instagram and creating several sock puppet accounts, then using them to post content (with comments disabled) bullying the user, with no recourse to defend themselves.
So basically your plan to shut up someone asking for anti-bully feature on Instagram is to give them plenty of examples of bullying conveniently located where Instagram not only sees them but also cannot avoid taking a position.
I can get past libel by just heavily-criticizing them on factual things--like their idiotic demands to restrict people's speech and give them no recourse to comment on images which assault their character.
You're going to get past libel because nobody cares how buthurt a pseudonymous Instagram account is about another such account's feature requests.
Also, they have the recourse of telling Instagram a stalker is making sock puppet accounts for the sole purpose of attacking them. Instagram, not being the government, can then completely ignore how carefully you skirted the edge of law and simply permaban you. Or, if they're feeling nasty, sue you for using their system in breach of the user agreement, which forbids harassment.
But, hey, who knows? Maybe companies forcing good behaviour online for business reasons will end up teaching the next generation to debate without resolting to volume or insults, thus saving our democracies. It's a longshot, but any port in a storm.