Curing Lycanthropy
The companions method
This answer is taken from the Elder Scrolls Wiki
WARNING: Once Lycanthropy is cured via the companions method, you cannot contract Lycanthropy again, nor do you have access to the Underforge. At the end of the Companions questline, a radiant quest "Purity" becomes available for those who wish to rid themselves of the disease. As part of that quest, you must cut off the head of a Glenmoril Witch, bringing it into the depths of Ysgramor's Tomb, draw the spirit of the wolf from your body, and defeat it. In order to remove the curse however, the Dragonborn must have killed at least two Glenmoril Witches, one for Kodlak, one for the Dragonborn. If you kill all 5 witches you can cure all of the The Circle, except for Aela who does not want to be cured.
The Vampire Lord method
With the release of the Dawnguard DLC another methondmethod has become available to cure Lycanthropy. All you need to do is get the quest Bloodline and choose to join the vampires. Becoming a Vampire Lord will remove the Lycanthropy.