Start out polite and assertive. Tell your admirer in no uncertain terms that you don't want this attention. Stay true to your word by discouraging contact with this person and refusing gifts.
Embarrass your admirer if necessary. It's always better to be polite when possible, but if your persistent admirer just won't take the hint when you behave reasonably, resort to embarrassment. Tell your admirer loudly that you're not interested, and make a scene about it so that everyone around--including your admirer--gets the point.
Change your location or your schedule. Sometimes minor changes on your part can distance you from your persistent admirer or eliminate unwanted attentions. If you always run into your admirer at the same times and places, vary your routine so that you're harder to track down.
Surround yourself with friends. If you're always with your significant other or a group of people, it will be harder for your persistent admirer to get your attention. Recruit your friends to your cause, and ask them to discourage your suitor too, so that you don't have to face the unwanted attention alone.
Seek help. There's a difference between adding a little variety to your schedule to avoid someone and rearranging your entire life out of fear for your safety. You do not have to deal with unwanted attention, and if your persistent admirer doesn't get that message, seek help from the appropriate authorities.