As far as I'm concerned spamming is when someone joins a site merely to promote goods for sale or another website. If, for example, someone starts posting what appear to be 'harmless' posts but they have a link to a sales website either in their signature or as their homepage, I would consider that spamming.
Note - this is not the same as a well established member posting a link to a site that they own that sells things - for example Mad Collector and chillicamper.
We also tolerate people posting to advertise Discworld related things such as plays etc.
I do not consider posting a smiley as spam. Sometimes a smiley says exactly what you want to say. For example; you don't need to write "that's very funny and I'm laughing out loud when you can just post this -->
As Q says there is commercial spam which we don't tolerate and then there are trolls - thankfully we've had very few of those here.