First, the bad news. I don't think you can hide or remove individual entries from a date-based search in the Mac OS X Finder. What you can do, however, is to hide the convenient "Today", "Yesterday" and "Past Week" search shortcuts entirely.

Does that solve the problem?

Well, kinda sorta. It's all I can think of, so perhaps someone else can offer up a solution that lets you edit the search results or hide specific matches from showing up...

Here's a typical Finder window in Mac OS X (I'm running 10.6.4, but it's been pretty much the same for quite a while):

I have the search options hidden by simply having the tiny triangle next to it pointing to the right rather than down, but if I click on the little grey triangle adjacent to SEARCH FOR:

Now we can see what you're talking about. If you Ctrl-Click on the specific item you want to remove -- if you do -- then you'll find that there's a handy contextual pop-up menu that shows up:

Rather than remove this entry, I'm going to choose "Open Sidebar Preferences...", which reveals the following:

Now you can see all your options, including the ability to hide "Today", "Yesterday" and "Last Week". That's the best I can offer, I hope it helps you out with your dilemma.