The Web host is very much in order. Unfortunately, most Nigerians do not read a company's Terms of Service before signing up. Stuff like that would have been clearly stated there and you agreed to it.
Spamvertising is the act of sending unsolicited emails to promote your website. The only evidence your webhost will need is a copy of the unsolicited email from the complainant containing a link to your domain name.
Email addresses can be faked and some email programs can allow someone use an email address that does not exist to send an email.
So, with such scenarios it will be very wrong for a web host to punish a client for something outside his/her control.