It's one of the lowest forms of spam, and I feel your pain.
Generally it's some idiot kid trying to get a rise out of the admin, maybe someone who plays on another board - or perhaps just someone who thinks roleplaying is stupid and wants a dumb laugh. This is why IP's are admin's friend.
And also why I love the delete button in the ShoutMix... having to go back to the website and log in is annoying. *sigh*..
We actually had an ex-member do it to us once. It was hard to ban that IP, simply because he had actually been on the board for six months, and I hoped he would come around... I eventually got an apology, but it really did upset me. But the bottom line is, whether it's a random or a member doing it, you've got no choice but to ban. Randoms you don't have a care for, never knew them never will...
.. but members like that, you don't need.
I was never too fond of Shout Boxes, because persay someone like what you are describing to us now.. or a mean ex-member always go right after the next easy target! The Shout Box! Where they can spam the heck out of it, create drama.. It's just so ugh and very frustrating...
However I personally think.. to avoid this in the future, that you should set the shoutbox privately to where only members can view it and post in it!
I had to do that recently with mines because of a few hahaha ex-members wanting to take advangte of pissing me off. Needless to say it only got me to be more smart ! ^^ But IPs is a wonderful thing too! Because ooo you can just do one thing to keep those nasty spammers off there and that is to banned them.
However if you have Shoutmix your not able to do that.. I couldn't do it... but there is another shoutbox that DOES have that and I do admire.. But I more so like Shoutmix simply because its easy to manage... and you have a lot features, but then again I bought it so I could have these features XD Bah! I'm rambling!
But I do feel your pain.. just remember to get these IP addresses and ban them immedately! Also see about getting it to where its only viewable to the members!