I use RecaptchaControl, and the users complain that the image is not that clear (the black part in the the captcha is too hard to read). Is there any property to make the image less difficult (less noisy)?
<recaptcha:RecaptchaControl ID="recaptcha" runat="server" PublicKey="XXX" PrivateKey="YYY" OverrideSecureMode="true" />
Possibly not what you are looking for but there are lots of custom captcha controls out there i.e.
CaptchaNET_2.aspx
With a bit of knowledge of the Graphics classes in C# it isn't a hard job to tweak the code to make the captcha simplier (or harder). We use one for a mobile phone web site and control is important otherwise it is unreadable in lower res phones
I'm the author of the article about the recaptcha API tutorial @web-development-blog.com and must say it's often very hard to read the text created images by recaptcha. While using the API I got sometimes better results while using a different language for the challenges.
In one of my projects I got a lot of complains and switched to a different script: http://code.google.com/p/cool-php-captcha/
The challenges are less hard to solve and the script is very easy to use.
In some reCaptchas (I'm guessing this is somehow configurable), you only have to get one word right (I'm not sure how close the second word must be).
I've tested in the reCaptcha in my own website and it worked (two words wrong = fail, one word right + one wrong = success); tested in stackoverflow's reCaptcha and it didn't... I had to get both words right. That's why I think it's configurable.
I found this post along with the one below (where I read that only one word had to be right) exactly because I too am finding it too hard to get through reCaptcha's; too often having to refresh more than 5 times to get a readable pair. Other post: Are reCAPTCHA CAPTCHAs getting harder or is just me
Google now allows you to change the difficulty setting.
To put it simply, no.
As others have said you can only customise the UI.