It's not possible I think to document the criteria as essentially Lichess can take whatever action they like, when they like and to who they like and they don't even need any evidence of a breach. Their terms of service
do give some examples though:
Fair Play and Community Guidelines
Users, both registered and unregistered agree to behave with good conduct whilst using Lichess’ Services. This will always be determined at Lichess’ discretion. Users who don’t behave with good conduct may have their account banned or closed without warning, and their content removed from the site. In all circumstances we reserve the right to ban or close an account for any reason without warning, and without having to provide evidence that Fair Play and Community Guidelines have been breached.
See the following for some examples of what you shouldn’t do.
1. Cheating. We define this as using any external assistance to strengthen your knowledge and, or, calculation ability to gain an unfair advantage over your opponent. Some examples would include computer engine assistance, opening books (except for correspondence games), endgame tablebases, and asking another player for help, although these aren’t the only things we would consider cheating.
2. Artificially inflating or deflating your rating. This is where a User purposefully loses, or has arranged with an opponent to win. As a result, the User’s rating will artificially increase or decrease.
3. Impersonation. Don’t pretend to be someone who you’re not. You’re welcome to use a pseudonym.
4. Trying to break into accounts of other users, even if it’s as simple as just guessing a password, DDoS and volumetric attacks. This activity is illegal, so the relevant authorities would also be notified. You are welcome to support Lichess by finding and reporting security issues to us.
5. Harassing conduct or offensive language. If you aren’t sure whether to say something, think about how an objective outsider might view it. Harassment is a criminal act and will be reported to the relevant authorities.
6. Sharing copyrighted material. On some parts of the site you can share your own content with the public. Posting anything other than what you’ve made yourself, or have a fiduciary right to share is illegal. Users are responsible for what they post and share.
7. Abuse our infrastructure. Don’t take any action that imposes an unreasonable load on our infrastructure or on our third-party providers. We reserve the right to determine what is reasonable.
8. Spamming. Posting unsolicited messages like junk mail, chain letters, or just general spam is just frustrating for our Users, so don’t do it.
These are just some suggestions of what we would consider against the spirit of playing fairly in our community. This list is non-exhaustive and doesn’t detail all activity we can or will take action against. The final word is always with the administrators, and penalties will be applied at the administrators’ discretion.