Add some debugging info to any events that have a high performance impact - loading chunks for example; if they’re doing that at a sufficiently high rate that could slow the game to a halt.
Next time you have this issue, you can try and spot patterns and see how the attackers is exploiting your game, and then either remove that avenue of attack or write a script to detect these attack patterns and ban the account.
First off, it’s not an edit if you make a new reply! Second off, a DoS attack is a denial of service attack. Effectively they are overloading your game with some sort of action in a way that the game cannot keep up with. Tom Scott’s Computerphile video " The Attack That Could Disrupt The Whole Internet" explains this.