I just kept muting the objects as I encountered them, but I quickly discovered that a lot of SLers- even ones who've been around for a while- don't understand that you can mute spam from objects just like you can from residents. If you didn't know you could do this, then the following steps are for you:
1. If you dont have the chat history window open already, click the History button on you chat bar. (If you don't see the button/chat bar, just press ENTER on your keyboard to bring it up.)

2. With the History window open, make sure the offending object is selected in the drop-down box next to the Mute resident button, and then click Mute resident.

3. Clicking that button opens up your list of muted residents and objects. Note that if people are talking in your vicinity, it's possible to accidentally mute them instead of the intended target. Make sure to check your list to see that the object/person you muted was the correct one. (If you muted the wrong person/thing, select the wrongly-muted entity in the list, and then click Unmute.) In the image below, you can see that 'COPYBOT SECURITY SYSTEM,' the object I selected in the previous image, was successfully added to my mute list.

NOTE: If you're responsible for putting anti-copybot devices out on the grid, please consider removing them. You're confusing newbies who don't know how to mute objects, annoying everybody else, adding a burden to SL's communication infrastructure, contributing to lag, and not really doing any good since anyone determined to maliciously use CopyBot has probably already muted all the defeaters- if they haven't re-written it to ignore the quit messages- by now anyway.
FURTHERMORE: To those of you saying that muting doesn't work against the CopyBot defeaters, you're obviously doing something wrong. Muting IM is half of the reason the mute function exists to begin with.