Lead with the decision
Thank them for the time, say you did not feel the connection you are looking for, and wish them well. Avoid maybe later if you already know the answer is no.
Do not provide an unsolicited critique
You usually do not need to evaluate appearance, personality, or dating skills. If they ask why, repeat the compatibility decision without inventing a fixable defect.
Choose safety over etiquette
If the person was threatening, coercive, or ignored boundaries, you can block without a rejection message. Use trusted support, platform reporting, or local services when needed.