Girls, how do you react when a guy asks you out, but you don't want to or are already in a relationship?

If a guy came up to you and genuinely seemed interested in getting to know you, but you were not interested for whatever reason, how do you respond in that situation? Do you appreciate it, get angry or annoyed, etc?
Girls, how do you react when a guy asks you out, but you don't want to or are already in a relationship?
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