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Maybe you should noindex user profile pages until they've been active on the site long enough that a) you're sure they're not a spam bot and b) they're actually active users, with some interesting information to share on their user page? Like Shaliendra said, this is your call. It's really about your website goals, and what those user pages do for the site. Are you getting much organic traffic from user pages? If you aren't, it's probably easiest to just noindex all user pages. But if some profiles are bringing in organic traffic, I'd recommend that you add a noindex tag immediately, but remove it once someone has commented a few times. Does that help?