How do you Get an Ex to leave you alone, when you are with someone else if you have children together?

If you leave your partner, you don't love your children any less, but some ex's make sure the children suffer, or use them as tools to hurt. Is there a solution out there, because I have tried everything with ex, from being supernice, to very cold (not with kids!)and am starting to have a major stress issue! If you do break up with someone, your children should always be able to see you as and when they need you, and know you are there whatever happens!