To better help you, can you elaborate on what it is that's being spammed? E.g. registration form, login form, mailing list registration etc. Would it be feasible for the user to click on a link in a confirmation e-mail before the action is preformed?
Don't use any type of captcha. In third world countries there are services were it costs $1 to solve 70 captchas.
Use a honeypot, where an input field is hidden so no user (i.e. actual person) would fill it in but a spambot would. If the form is submitted with a non-empty field then you know it's spam.
There are other measures that can be taken too. ASIRRA, by Microsoft, is an alternative where you select pictures of cats or dogs and if you selected all correctly you are not a bot.