If you put the captcha, any Xrumer it will pass. If you really want to kill bots, then change the name of fields, for example where to enter the address there set name="nameUser", and where the name is there — name="email". Of course the bot will stumble on this exactly and the drum.
You can also use hidden fields, for example name="url" and style="display:none;". If anything is introduced it is sent, if not then everything is OK.
In addition to the above two methods also use a timer (well, not a man-anonymous comment, enter the captcha and fill the name field in one second).
Spam has not seen before two years in the comments from anonymous.