When someone I don't know and I did nothing to (easy actually, since I never bully anyone, unless retaliating) is attacking me (usually bullying my emporia), I raid them, then I raid their hegemon, as he's reponsible for telling them not to attack people who are not ennemies, and if the message isn't clear enough, I go on raiding their polemarches, emporia, etc. until I get a reaction.
Some understand, others don't... However, the wise reaction for their leaders should be to enquire to the bully and blame him. When you can't fight a bully (usually hiding his troops because he has planned his crime), you can still apply pressure to his team mates. When they're unaware, you can do huge damage to them, and trust me, they'll move.
You can't imagine how a lone player who has nothing to lose can be a pain for them. They'd quickly rather kick your offender than going on standing your wrath.
So use a mix of dialogue, diplomay and retaliation. Don't care for insults, just despise him and attack his mates. He'll quickly swallow his insults. Even if he gets kicked and goes on, go on doing the same thing to his new coalition. He'll get tired of this sooner or later (and he'll probably insult you even more, but you can block him ^^ LOL).
Don't cower, show your teeth instead. Your nuisance capacity is higher than you imagine, especially if you get others pressure your bully for you.
Also always message his hegemon and explain him he's got nothing to earn allowing your bully to go on. Ask your hegemon or embassador to discuss with them. Problems like this are better fixed by diplomacy anyway, but striking back is also part of the diplomatic process.
Your coalition is also supposed to help. The best way is to ask your mates to pile up millions of defense in your city walls so your bully smashes his offense against them. He'll definitely insult you, but you'll feel how good it is ! ^^ LOL
Your losses should be extremely diluted and you'll benefit from 30% free revival, while 30% of your bully's troops will be lost forever. So even if he's a coiner and can revive the remaining 70%, trying again will make his army melt like snow in summer, and the most he'll insist, the faster he'll end up totally naked and harmless. :p
And finally, adopt a faultless behavior. Never attack anybody's city nor emporium unless he's a declared ennemy. Never hit any coalition pantheon nor capital unless they belong to declared ennemies. Never attack anybody unless he's an ennemy or you've agreed to fight. Ancient Greeks had honor and declared war instead of sneaking treacherously.
You have more chances to get positive attention from your bully's hegemon and coalition members if you have not attacked them for no reason before.
Because if the guy is bullying you back because you have attacked him first, and especially if you have killed his troops and made him lose his time and resources at an emporium, then YOU are the bully, and nobody will support you...