Anyone know how to do this?
The easiest way is to stop letting people who are not you use your account on the computer.
If someone else wants to use the computer, create an account for them or let them use the "Guest Account."
Sharing your account is bad ju-ju and is begging for problems. You and only you should use your account, and it should be protected with a reasonably strong password.
...but what happens when someone pops their head into the room and says, "I just need to look up [blah blah] on the internet, real quick!" Simple: simply activate Fast User Switching to the Guest Account and let them look it up.
In addition, the data that is used to track what you've used "Today" may not be "untrackable" -- for instance, if you created a Smart Folder that tracked all files created in the last two days, well, that's something that can be found outside of a Smart Folder, too -- by the file's creation date. You cannot change a file's creation date. Similarly, you cannot change a file's modification date -- which is most likely what the Smart Folder "Today" is using.
I don't think there's any hidden metadata or special "Mac OS X-specific" data that's being tracked anywhere -- I think the "Today" folder is simply looking at files' creation and/or modification dates, and those are things that you cannot get rid of.