Spry has nothing inbuilt that will do what you want...you are going to have to do it manually.
The easiest way is using the 'honeypot' technique. A hidden form field which if filled in stops the form submitting the data. Supposedly spam bots fill in all of the form field BUT I'm now skeptical as to if this happens as I have had a client report some spam which used this technique:
Here is a jQuery example/tutorial of the honeypot technique:
http://www.dexmedia.com/blog/honeypot-technique/
Captcha in my opinion is much more secure but a lot suffer from being too distorted to read making it impossible for the user unless they are prepared to have half a dozen goes, hardly user-friendly.
You could set a question like 2+4= and check that the answer is correct before the form is submitted but you would need to know some server language to do this.