Is this cheat where somebody puts up that warning to kick somebody , and it keeps coming up and down very fast ?
Actually we see "flashing" players all the time in long games. What is happening is that their connection has gone bad but not completely away. Since these players are almost always people we know well and know not to cheat it's attributable to a bad connection.
Some things that can cause a player to lose connection:
In long treaty games, or even in short games if someone has been on the Internet for a while, some routers and modems will heat up and lose connection.
Other times it's probably the Internet system itself. For instance the Internet in Mexico, Brazil, Russia, and China often acts up and drops players from the game.
Other people accessing the same network that you (or another player) are on may have higher priority. If you have access to your router setup page you can give your computer and the game higher priority to prevent this.
Bad weather -- I've lost more than one connection during the tornadoes we have around here -- if one crosses the right power line or Internet cable, well that's all she wrote for that game.
Windows update -- sometimes several computers in a game have automatic updates happening at the same time, some updates even reboot your computer, thus kicking you from the game.
Malware. If you don't have a good anti-malware program and tend to open a lot of attachments and surf sites that cater to porn, file torrents, and music lyrics (for some reason these are often infected) then you may have a Trojan that is using your computer to send thousands of spam e-mails every time your computer is connected.