Anyone found posting or reposting chain letters in video comments will either be severely warned or have their account deleted without warning. In most cases, the accounts that are to be deleted will be the ones that have very few CR points and haven't really interacted with the site anyway. I would warn each one of them beforehand but there are too many and it would take to long.
These people are welcome to recreate their accounts right afterwards, but obviously should NOT post any more chain letters or spam messages.
Just use some common sense when making comments on videos or on the forum.