I very highly doubt Valve actually has systems to check the bytecode the SM VM is running (which in turn can be used to determine what plugins are being used). They likely are taking the "we'll ban servers with the method that has the highest accuracy:effort ratio". (So piss poor accurate detection methods that took them barely any time to implement are probably being used. At least that would explain how some servers never got banned while others were wrongly banned.)
Empty out your tags and change your hostname to make it not obvious you have knife givers. They may have some regex scanner running through the master server every now and then.
As one additional precaution step, on all economy given plugins, change the data in "public Plugin myinfo" to something that makes it not obvious it's a knife or skin plugin so that typing "sm plugins list" doesn't return anything that looks like a knife plugin is installed. I doubt Valve looks at that output but it is another easy way to catch your server on fire. If you block off all the obvious places that Valve can easily check/query your server for with no effort (hostname and tags cvars being examples), you likely will avoid the blacklist ban.
Think like Valve. "What is the cheapest and easiest way to ban servers with knife givers [within 50% accuracy] with the least amount of effort on our part?" (First 2 answers should be checking hostname & tags.)