banning will not help your spam problem. IPs change all the time. the IP address you have today may not be the same one you have tomorrow etc.
banning via email or username is also not going to help.
the way to stop the spam is not let them get past the registration to begin with.
use the builtin Q&A with a question that you can not find the answer to in google.
set the first post to need approval.
you have then solved at least 99.9% of your spam problems.
as has been said many times, the main point of the contact form is so people can contact you when they are having problems registering. in other words when they can't figure out the anti spam measure you are using.
if you are also using anti spam like Q&A or recaptcha then they won't be able to figure that out either and won't be able to contact you for help.
yes, spamming the contact form is a problem. there are at least two different extensions that will solve it.
doesn't matter if you think it should not be done with an extension. if you want to solve your problem use what is available to solve it.
I am available for custom work on a donation basis. Please send me a PM with your needs.
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OK, so what's the speed of dark?