The current rules come from the Gitcoin Holdings Inc. Terms of Service and the established norms of the community. The next step in progressively decentralizing is to have the stewards of GitcoinDAO ratify the entirety of this policy.
Users are not reviewed upon activation on the platform. They are only reviewed when one of our detection mechanisms flags them as "presenting as fraudulent or sybil".
Users in violation of the ToS or norms & values of an ecosystem may have their contributions effect on match allocations diminished or turned off. They may also be blocked from accessing the platform.
At each level, the participation policy allows for a subset of the users allowed at the higher level.
We cannot possibly enumerate every single literal example of a breaking of the above rule, so please treat the above list as an incomplete list. You should refer to the definitions of Sybil and collusion attack in the CLR paper as the canonical source of truth for the rules.