That dialog is particularly annoying and persistent. Providing a valid password is one way of stopping the dialog from coming back, though I find that when this occurs it is best to do the following:
- Log out
- Log back in
- Enter a valid password the first time this dialog pops up, OR enter the valid password in the iCloud page of System Preferences.
If I enter the password on the second, third, forth, etc. request, It seem that not all the things that wanted it get access to it.
If the problem is that you don't know the password, and have no way of recovering it, you might try deactivating or deleting the iCloud account from the iCloud page in System Preferences, then restart or at least log out and back in. You can always add the account again later when you have the correct information.