Ref spam is a form of vandalism in which references and other external links are added not to improve MEpedia, but to drive traffic to a particular external website. Typically this is an issue when it involves the addition of unreliable sources. This can take place manually or by an automated mechanism like a bot.
If you suspect ref spamming may be occurring, first assume good faith. A new editor may simply not realize what constitutes a reliable source or other referencing guidelines. Consider leaving them a message welcoming them to the site and offering guidance.