Act like a spammer, get treated like a spammer. www.rxrefillshop.net is one of many websites that try to spam Skype users.
An interesting test would be to set up a 100 Skype users with predictable common names (john smith etc) on virtual machines and monitoring the URLs received in messages. I suspect there would be few false positives in the spammy URLs that would be received. These URLs would then be added to a potential spam database. Methods of spam identification that track IM and Skype would also disincentivise spamming of these communication channels.
A relationship between Skype and Search engines would be even more efficient, whereby URLs send more than n times to people not in the users contact book (and similar spam signals) would have the URL automatically flagged and communicated to search engine spam teams.
Skype Webspam
