Summarize the following:
If they are behaving generally rational, your ex's advances are fueled by contact with you. Don't give fuel to the fire. Try to avoid them whenever possible. This may be difficult if you work together or socialize in the same circles. Nonetheless, the more you can break off contact with them, the easier it should be to gradually come to terms with the break up. Take note if your ex intensifies their attempts see you after you break off contact. This irrational behavior may be a sign of criminal harassment or even psychosis. If they become more desperate or abusive, contact the police immediately. It is important that you have a support network in times like these. Keeping others aware of your ex's behavior will also serve as an impartial judge of whether they cross the line. Have them check in with you regularly to make sure you are okay and the pestering hasn't intensified to abusive harassment. If they leave you messages/texts that make your blood boil, take a moment before doing anything. It is probably best not to respond at all, and let their feelings for you suffocate. Alternately, you may think that will only make them more determined to getting in touch with you. If so, keep your comments and responses as positive, respectful and brief as possible. Communicate to them that you respect them, but no longer wish to have a romantic relationship. If they won't stop texting you, try a quick message like, "I still respect you, but I don't want to date you. Please stop sending these texts." Until this blows over, you may want to avoid seeing their correspondence. Most email clients make it easy to filter out unwanted messages. However, if you don't even want to be confronted with the possibility of opening their email, you might create a new temporary email account. Share the address only with those you need to contact, and tell them why. Making sure your ex doesn't find out this email address will create a safe space for you. For a step-by-step guide to creating an email filter, see this. Their phone calls and voice messages will be harder to ignore than their emails. This will be a major source of stress for you until the issue is resolved. Luckily, it is easy to block phone numbers. Look at this guide for instructions on how to do this with a variety of phone carriers. If your ex is part of one of your circles of friends, take this as an opportunity to explore new forms of entertainment and cultivate new friendships. It is unlikely that all your old friends break off contact with you unless they have a reason to. Nonetheless, take the stress off of your interactions with them by keeping a safe distance. They will appreciate you keeping them out of the dispute as well. Sometimes you cannot avoid direct contact with an ex. Try to keep the contact brief and public. They are unlikely to do anything abusive with others around, and it should make you more comfortable. If they do cross the line to harassment, others will be around to step in or at least act as witnesses. Acting stressed or hysterical may make your ex feel like they need to 'help' you. By having calm, public interactions with them, they may see your happiness and not want to disturb it. After all, if they truly cared for you, they would want you to prosper even if it meant leaving you alone.

Summary:
Avoid them. Tell your friends and family. Don't respond to their communication. Create a new email filter or account. Block their phone number. Socialize with other friend groups. Keep interactions short.