How can you handle griefing in online games?

This is the big must, the more detail you can make your rules the fewer loopholes others will try. Does it work 100% of the time? No, people who think they can get away with it will always try it but hopefully, by having said rules and communications open, others will report them. Having them in multiple places help too, Forums being the big one but social media and websites where people check can be your platform to express concerns, why such actions are griefing, and how people can help one another should that defined toxic player be at it again.

Yes, people will report others if they don't like how they play or behave (specially if you try to offer help and they think you're insulting them) -but before throwing a ban hammer, be sure the report is accurate. The best example close to this is from the FFXIV game where GMs will pull players online to the 'jail', and explain what has happened, what was investigated, and what results were taken. They do however give you the ability to appeal this decision. Holiday bonus story but there was one game that gave users Christmas presents. If players open the box and are nice, they get gifts but if they are 'naughty' and the behavioral system shows this, they receive coal and a ban (they did go through their data making sure of this)

A MOBA called Dawn Gate had a really great solution to this. They tied player progression/power to being a polite player. When the player would finish a game, they would have the opportunity to vote on if other players were good sports or not. By getting more votes after a match you would gain additional XP and rewards. If you were a greifer or a bad sport, no one would vote for you, and you wouldn’t get the bonus. Thus the player who was friendly would progress at a normal rate, and players who were rude would progress at a dramatically slower rate. It resulted in a polite community.

Dealing with griefing in games can be as simple as positively encouraging good behaviour. Take 'Overwatch' as an example—the game rewards players for praising others' good conduct. Similarly, implementing a system where players are recognised or rewarded for cooperative actions can be effective. Encouraging teamwork, like in 'Among Us,' where players rely on each other, naturally discourages disruptive behaviour. Another potential approach which works is to incorporate limitations similar to 'Fortnite,' which restricts certain actions to prevent griefing. Additionally, empowering players to moderate, like in 'Minecraft's' player-driven servers with moderation tools, can create a nicer gaming environment.

WYSIWYG (good old fashioned What You See Is What You Get)!! Do not promote/advertise a game offering things that cannot be delivered. A clear view of what the players get minimises complaints that can lead to griefing.. Always have a clear structure and be flexible at the same time. That offers stability but at the same time it gives the extra edge that the players are looking for which in return makes up for happy people with smaller percentage on the negative side.