What to flag as SPAM
Flag any answer or question that you think violates this policy: if the content is flagged by other users it will be likely edited (if the SPAM is only a secondary part of the content) or deleted.
To check if a link is dangerous or SPAM, open it in the Private Browsing mode of your browser or download it with
curl to read the code with a text-only editor (local execution of js scripts may circumvent some protections).
Some advices to help the users
Be very careful with the users that report a virus infection in Ubuntu, because they often download a Windows(R) malware and ClamAV reports it even if it cannot infect a Linux machine. In all cases, the reported file should be deleted, because a network may also contain Windows machines and nobody wants a virus/malware in the monthly backup.
Also, some websites try to convince the user that the system is infected and offer a download that hides a trojan horse. Some may infect both Linux and Windows by using social engineering (or memory exploits) to get the administrative credentials and portable languages like Python, PHP or Java to cross the barrier of executable formats. Just ask the Ubuntu user to delete any copy of the infected file, disable suspect browser extensions and never visit that web domain again.
I'm trying to be helpful but my answer is far from being complete and my English is weak. Feel free to ask in the comments if you have more doubts. :-)