The first step that should be considered is that you need to figure out what is the reason for your ex's actions. Consider that they may be going through a normal process that you may find someone undergoing shortly after a breakup, or that they have a problematic obsession. Think whether or not that you feel intimidated by your ex, do their actions seem to bother or annoy you? If they show to create these feelings than you need to consider a way of dealing with it.
If you are uncomfortable with your ex, then look to setting guidelines for them. Look to what is bothering you and look to doing something to stop it. If they seem to be annoying you, then calmly inform them, in a mature manner, that they need to quit whatever action that is annoying you. Remember that you need to be respectable to them regardless of how they handle it. If they choose to be immature about your request for them to stop, then allow that person to continue and learn the hard way.
If they seem to be reluctant to any request that you may have, then look to a friend to attest to their actions. If your friend is more than capable of describing an instance that you could use to help emphasize your request, then do so. Make sure that you are certain about how you feel about your ex, because there isn't a point of making a problem out of something that is simply a misunderstanding.
Be sure you are respectful to your ex's feelings when you look to taking action to something that bothers you. If you choose to handle it immaturely, it will only make whatever the problem is a much larger one.
Resources:
How To Know If Your Ex Still Loves You
