One could think of presenting the same information in different formats: everytime the next page is presented the lay-out is somewhat different: can be as simple as switching zipcode and city around; or moving the contact address to the front and the link to the picture of the house to the back of the row; or ... This would certainly annoy any automatic harvesting of your pages.

It'd also likely annoy regular users of your site. Imagine if the PM voting buttons moved around (sometimes above a node, sometimes below, inconsistent order, etc), or if the nodelets' positions couldn't be guaranteed.
Humans are pretty good at spotting differences in information if the information's layed out in a consistent manner. If you're comparing properties, those differences (price, number of bedrooms, city) will be all important, so making them harder to spot will also make your site harder to use.

Or include "invisible" records (e.g. background and foreground color the same and very small point size) with good-looking but bogus information, which would then poison the harverster's database (although this will be bad for text-only browsers).

I'm nit-picking now, but not just text-only browsers. What about people using high-contrast colour schemes in their browsers? I genuinely believe the OP has a difficult task if they want to preserve their goal of accessibility.