This addon filters the trade channel and /yells (and all other numbered channels, i.e. /1 through /99) so that any individual message is only displayed ONCE every XX seconds, where XX is a user-settable number with a default of 600 seconds (10 minutes). Repeats are filtered out, as long as the text is similar and it's been less than XX seconds since the last identical message was shown. Repeated spam typically happens when someone has made a macro advertising something and repeats it over and over - this addon ensure that you will only see the chat message once only, unless the person changes their script or macro, in which case the new message is displayed once only. You don't miss any messages, just the duplicates are removed. A fuzzy match filter is used so that even if text is only similar it can be identified as a repeat. The addon is extremely lightweight and is completely self contained.
All of the chat channels and /y are monitored, but non-numbered channels (like /g or /w) are not. Any message repeated on any monitored channel is filtered, even if the message appears first on one channel and then the exact same text is repeated on another. On a very busy server, repeated advertisements for groups and trade items can make the chat channels nearly unusable, as they are constantly scrolling with hundreds of messages per minute - but removing the duplicates can reduce the channel chatter by as much as 80% depending on the server, thereby reclaiming the usefulness of the channels.
The spam can be colored dark gray so that it's easy to ignore (and also so that you can see what the addon is catching), or can be hidden completely. You can reset the filter altogether which clears it's memory and starts it from scratch - allowing you to see all new messages that appear (once) as the filter restarts recording what has been sent, or just depend on the gapping value to show repeated messages every XX seconds. The addon has a default setting to hide all spam; to change to 'color' mode use " /st color " and to change back to hiding spam use " /st hide ". You can also turn off the fuzzy match filter so that only exact matches count, using "/st nofuzzy". Turn it back on using "/st fuzzy". And you can turn on filtering of messages containing Chinese, Japanese or Korean characters with "/st cblock" and off again with "/st nocblock" - default is off.
Credit goes to Yewbacca's ASSFilter addon, which was used as a base for some of the code here.