I was a little dubious if this would work as we were getting between 15 and 20 spam emails a day through the Contact form and tbh it was getting a bit tiring adding their IP ranges to the block list as it didn't seem to make any difference, the numbers still kept coming.
Having installed this add-on it has killed spam from the Contact form stone dead, not one has been received in the past 24hrs.
I have just started testing this on three forums I manage and I must say it is very impressive and very effective. Contact Form spam has always been the weak point in Xenforo's anti-spam features and nothing else I tried was able to stop it. Contact Us Spaminator works as advertised. In less than 24 hours, it has blocked more than 30 contact form attempts on the three forums. I don't own the three forums where I am testing it but I will be encouraging all of them to buy a license and certainly the first 24 hours is convincing evidence that they should do so.
It was installed late evening October 17. We are now up to 126 contact form spam attempts blocked on the three forums in just about 48 hours. I can definitely recommend this addon.