I have written on sandbagging in several other threads, because I spend a lot of time dealing with it, so some of this post may be familiar to regulars here. I am going to keep this informational and not try to address specific points by previous posters. Sandbagging is a huge problem on all go servers, and non-moderators have no idea of its scope. There are two types. The type that most players know about is rank manipulation, where players throw games to weaken their rank. This is the kind that players can see by looking at someone’s game history. The amount of sandbagging done like this is trivial. The other type, which players cannot see, is alt sandbagging, in which players create a new account, which is obviously provisional to start. I estimate that alt sandbagging accounts for 95% or more of sandbagging. Of course, not all alt accounts are for sandbagging. If the account has a special purpose (such as speed or board size different than the main account), then it is legitimate and may rank up normally.
The mod team investigates all reports, including all sandbagging reports, and I proactively ferret out alt sandbaggers by various means. Lots of players file SB reports because they can’t believe they lost to someone weaker (probably more than half of all SB reports). In most of those mistaken reports, the reporter (usually DDK) lost because of one or more very bad moves. In some cases the opponent is much stronger than their rank because they are a strong player who has come here from another server. I wish they would rank up by playing bots, or put their rank in their account name until they rank up, but that is not required.
I ban sandbaggers almost every day, sometimes dozens of alt accounts. Here are the protocols I follow: All sandbaggers get one warning. Thrown games and non-games (those with just a few moves) are annulled. Alt SB accounts that haven’t already ranked up are banned immediately, because they have no right to exist in the first place. If someone continues to rank manipulate or to create more alt SB accounts, then all their accounts and all future accounts are banned.