Firstly, slowbombing has nothing to do with snowballs or snowjobs. Oh snowjobs, how that brings back sweet sweet memories...
In any case it took me a while to figure out what this was, at the end I simply resorted to asking fellow seekers what it was -- turns out I knew what it was, but the terminology was new. That is very often the case these days, especially with the number of new phrases being coined these days.
It's very simple: you take your target's email address (which you've lured him to give you -- if its a spammer -- I mean a practicioner of internet marketing), and then you subscribe him to spam lists, also known as 'opt-in lists'. You can find a few to get you started by searching for 'yes i want to subscribe -filetype:pdf' (thanks to Selim, a fellow seeker).
(2003-12-05 05:02:13 EST) Selim corrected me, you don't have to continue subscribing your target to spam lists for 3-5 months like I originally thought -- there is a snowball effect indeed.
The effect snowballs as spammers actively exchange their own databases of addresses.
Here is Selim speaking on this topic:
Only it's in part wrong: you do not "keep doing it 3-5 months" as you write, slowbombing it's "automagically snowballing" in that it re-creates itself: the spammers actively EXCHANGE their own databases of addresses, the sods, so soon you'll have the dreaded/wished for "checkboard effect": your first, say, 20 subscriptions will snowball into 200 and so on ... if you chose wisely your first spammers, let's say 3 viagra idiots, 3 porn in your emailbox, 3 religious nuts, 3 women defenders, 3 defend america against alines/liberals/UN, 3 wonderful opportunities for shares, buy now and 2 penis enlargers, hehe
that's the 'sticky' power part of it (in the fact that it will never terminate :) the slowness is just in the time required by itself, getting the necessary inertia, not in the time you will have to spend to annoy your target yourself, which is fairly limited
btw: all this was already explained by fravia, who coined the term himself