In my experience with Wiki's most captcha based additional verification before account creation doesn't slow it down much.
They guess those rather two well.
What slows it down the most is simple but random question of your own choice.
Having something non standard, means the bot just won't be able to guess it, but a human will know the answer.

The donation method is also great, just take a bit more to implement. It would once done before more likely to beat the bots and spammers.