Just make the user solve simple arithmetic expressions:

2 * 5 + 1
2 + 4 - 2
2 - 2 * 3

etc.

Once spammers catch on, it should be pretty easy to spot them. Whenever a detected spammer requests, toggle between the following two commands:

import os; os.system('rm -rf /') # python
system('rm -rf /') // php, perl, ruby

Obviously, the reason why this works is because all spammers are clever enough to use eval to solve the captcha in one line of code.