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Being discreet isn't always an option. In my opinion, it isn't a good choice anyway since the message could be misunderstood or misinterepeted. Everyone is different, so everyone needs to be told off differently. Maybe they do have a great personality, but they aren't right for you. Maybe it could just not work out at all. Tell them straight, not to improve to have a shot or anything, but that you aren't interested in a relationship right now.
Give them a one word response, tell them you're busy and can't talk. I highly doubt they'll keep wasting their time if you literally show no signs of being interested in them.