OK - this is two separate things.
1) the numerous people browsing your forum are Search Engine spiders, looking for stuff to eat

<= if you are using vB3, there is a facility to distinguish these built into the AdminCP > vBulletin Options > Who's Online display > Spider display <- turn this on
2) the emails you are getting are a part of a global phonomenon called a pain in the a$$ spate of viruses ... <= nothing can be done apart from getting your email server to block email sent to addresses that are not actual addresses you have set up ...