While it’s important to make it clear that you are single and that you are “OVER” your last relationship, it’s natural for an interested party to wonder and question about you ex and the last serious relationship you had.
Assuming the question is about a fairly serious relationship, what I do, is create a time line that indicates it’s been enough time that I’m over it and ready to move on to another relationship. I might say, “We split up about 6 months ago, we were together for about 9 months, it didn’t work out. Someone once gave me great advice; it takes at least four seasons to get to know someone. I don’t think there is enough room in this relationship for 3 people, and right know I’d like to focus on getting to know you.”
Typically the other person will be flattered, and agree to move on, however if they remain curious, that simply means they are really interested. Most people will be, but will refrain will be respectful and refrain from additional question until a later date. If they bring up the subject again, suggest that you will happily discuss this on another date, but right now you are enjoying getting to know them.