It's a problem with anything depending on how you setup your thank you page
Probably not the answer you wanted, but I'll try and explain...
If you are using CGI, PHP, ASP scripts, then with a little clever code, you can make your pages more or less totally inaccessible without going via your payment link.
If you are using HTML then you need to be careful
First off HTML pages can get indexed into search engines.
For the search engine to find the page, there must be a link on some other page. So provided there are no links anywhere to the page, except via your clickbank link, then you don't have to worry. If your thankyou page was previously linked, maybe you want to rename it.
Next, some people call their thankyou page obvious names
It doesn't take much to guess this! So instead call your thank you page something like (and change it once in a while)
Next, some people suggest you create a robots.txt file - definition herehttp://www.searchenginerocket.com/basics/robotstxt.html- to tell search engies not to index your thank you page. Don't do it!. First off some search engines may index your thank you page because they see it robots.txt - not all search engines are well behaved! Also a clever thief can guess you have robots.txt file and type into their browser
and hey presto they know the thankyou URL!
Instead of robots.txt it is much better not to link to the thank you page anywhere (so it can't get indexed), and as I say change it once in a while.
Also you need to check your have a default file (usually called index.html or index.htm or UNIX web hosts, or default.htm or default.asp) in each folder on your site. Why? Because many web hosts enable directory listing if a folder does not contain this file.
For example, on many web hosts,
will list all the files in your site - ouch!
And lastly, check the last 2 URL variants with a period . on the end. Some (very few in my experience) web hosts enable directory listing if you do that. if your host does, ask them to change the setting - and if they don't - it's time to change web host.