best approach is what @scottalanmiller said, delete account but keep topic and post maybe with and alias user465 with the id of that account
another option should be the current one, delete all
Merge all topics from a category in 1 topic
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_ _ __ __ _______ _ _ _____ _ _ _____ /\ | \ | | \ \ / / |__ __| | | | | |_ _| | \ | | / ____| / \ | \| | \ \_/ / | | | |__| | | | | \| | | | __ / /\ \ | . ` | \ / | | | __ | | | | . ` | | | |_ | / ____ \ | |\ | | | | | | | | | _| |_ | |\ | | |__| | /_/ \_\ |_| \_| |_| |_| |_| |_| |_____| |_| \_| \_____| _ _ _ _ (_) (_) | | | | _ ___ _ __ ___ ___ ___ _ | |__ | | ___ | | / __| | '_ \ / _ \ / __| / __| | | | '_ \ | | / _ \ | | \__ \ | |_) | | (_) | \__ \ \__ \ | | | |_) | | | | __/ |_| |___/ | .__/ \___/ |___/ |___/ |_| |_.__/ |_| \___| | | |_|
But seriously, there's no easy or out-of-the-box way to do it that I know of.