There is no real way to completely stop these e-mails (known as spam). If you simply delete them from your inbox rather than replying to them this often helps. Many companies put the "Reply to xxx to be removed from the list" at the end of their e-mails so they can see which e-mail addresses are valid. The valid ones then get distributed to other companies, etc.
Any mail entering or leaving the department passes through the university's central email scanning system which checks for viruses and adds header tags which indicate whether it thinks the mail is spam or not. You can thus use this as a means to filter likely spam into a separate folder. Once in the separate folder you can check it regularly and delete unwanted junk quickly. For information on how to use these tags see the mail filtering page.
Also see the University junk mail page