[Plugin: Donate Plus] Donor Website Causing Spam without Donation

I’ve had to disable the ability of the donors to include their website address because I’ve had two “donors” post their websites without making a donation. Is there a way to require a minimum donation or some other way to resolve this. I would like legitimate donors to be able to include their website address, but not spammers.

http://ww.wp.xz.cn/extend/plugins/donate-plus/