The iPad is pretty much a mirror of an IMAP or an exchange account, or a download site for a POP account. There really are no filters to set up in the Mail app; it's mainly (and basically) for viewing e-mails. There's no real management of them.
Your best bet is to go the the web and create filters and such at the source (e.g. Gmail) or via your ISP. That way, spam will get stopped before it gets to the iPad.