Firstly, consider whether you're using a reasonably 'spam-proof' email address. Spammers set up their computers to send mail to millions of addresses which might
possibly exist. For example, they'll send mail to
[email protected],
[email protected],
[email protected],
[email protected], etc, etc, in the knowledge that at least some of those addresses will actually exist, But they almost certainly won't send spam to addresses like
[email protected] or
[email protected]. So it makes sense to use an 'non-guessable' email address.
Then consider how many different email addresses you use. As well as guessing at email addresses, spammers obtain real email addresses by hacking the servers of websites such as this one. (NB: I'm not suggested that AB has actually been hacked, or that it's actually vulnerable to any such attacks!). You should never use your 'main' email address to sign up to web forums or to order goods or services online; you should always use a 'disposable' address (which you can simply stop using if it starts attracting spam). An excellent email service for getting new email accounts (which can either be accessed through the web or with an email client) is GMX:
http://www.gmx.co.uk/
Lastly, to deal with your current problem more directly, you have to accept that spam addressed to you will arrive on the servers of your email provider. (There's no way it can be stopped). Some email providers have quite aggressive filters in place which will see spam dumped into a 'Bulk' (or 'Junk') folder on their servers, so that you'll never know that it existed unless you access your account using webmail (rather than an email client) and inspect that folder. (The only downside is that there are occasionally 'false positives', meaning that genuine mail might get blocked).
For people who only use webmail there are no further measures that can be taken (as accessing a webmail account is directly inspecting the contents of the email provider's servers). But those who use email clients can take a further step, which will prevent spam from being forwarded to the inbox on their computer. That step is to install software which 'learns' what is, and isn't spam, and which allows users to inspect what's on the provider's servers before choosing whether to download it or not. Fortunately such software is free:
http://www.mailwasher.net/
Chris