Unsolicited e-mail, often of a commercial nature, sent indiscriminately to multiple mailing lists, individuals, or newsgroups; junk e-mail. [1]
At Stumbleupon, what spam is and isn't is often in a grey area. For example, many people publicise their new forum they created to people on their friends list; this is not considered spamming, but some people do not take kindly to this. If you do not wish them to PM (Personal Message) you with this type of thing in the future, just try asking them politely to desist.
However, any such PM that is non-targeted (i.e. he sends it to everyone he sees), commercial and is not to do with Stumbleupon is safely considered spam. This includes PMs such as:
"... I would love for you to visit my homepage/forum at ..."
If you would like to report spam, use the feedback form: Feedback Form
If you are in doubt, see this thread: Spamming each Other