It takes a very ballsy woman to venture out to the club alone. Women tend to flock nightspots in groups. These she-wolf packs give rise to the cockblocking phenomena. A cockblocker is a female friend who accompanies another female friend to social gatherings or planned meetings with a male peer to ensure that the friend does not 'get into trouble'.
A cockblocker is that hovering friend at the club that whisks away your love interest every time you try to get close. Every man who is actively trying to meet women has had to deal with a cockblocker. A cockblocker might try to cockblock you for a variety of reasons; out of pettiness because she finds you distasteful, or out of a protective instinct to prevent her friend from making a "bad decision."
She might disapprove of you just from a glimpse and make up her mind that you are not good enough for her friend. In rare circumstances, she might be attracted to you herself and be jealous that you have chosen her friend over her, or she might care so much about you but is spiteful that her friend has met someone while she herself has not.
The most common reason why men are cockblocked in clubs is the friend thinks that her pal is incapacitated. If a girl thinks that her friend is too intoxicated to make sound decisions, she will most definitely intervene when you make a move to prevent her buddy from making a "mistake".
It is not only incapacitation from alcohol that can spur a cockblock into action though. The friend could also be emotionally incapacitated. Maybe the friend was recently dumped and may be behaving recklessly. Because of that, the cockblocker takes it upon herself to prevent her from doing something she might regret, like going home with you.
So, how do you deal with being cockblocked? It depends entirely on the reason you are being cockblocked. If you are being cockblocked because you are trying to pick up a drunken girl, just accept your fate of blue balls and move on.
Her besties know her better than she knows herself, so if they feel that she is not being herself and is doing things she would never ordinarily do, there is no chance in hell she is leaving the club with you. Sisters look out for each other. They will take whatever measures they have to, even if it means prying her out of your arms.
With the exception of the above scenario, you can easily get past a cockblocker. If the girl you have taken a liking to brings only one girlfriend, you can bring along a badass wingman to distract the cockblocker while you make a move on your girl.
Be cool and polite and don't be overly sexual and physical, otherwise her friend will write you off as a creep and step in to rescue her. Friends are much less likely to interfere if you come across as a decent guy.