It sounds like you want to block this port because this vulnerability exists. The real vulnerability here is not that port, but the user who executes 3rd party application sent to them over AIM. If you educate them you will be able to avoid a large number of risky behavior instead of protecting one port. If you protect against one backdoor like application there will be another using a different port.
If you are really concerned about this and you are on a personal computer I would recommend that you get a nat router. The NAT router will give you firewall protection as default ( as long as you don't open any ports in the router) and it will not impact your computers performance. Also cost is about that of any non-free firewall. There are a number of free firewall apps out there that you can use, the drawbacks just the impact on computer performance, and for the firewall to work correctly it has to integrate into the OS at a low lvl.
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