Not a riot, perhaps a lady in her 40s would just go up to the offender and pointedly make it clear in sign language that what she was doing just wasn't acceptable. More like acute embarrassment for all concerned.

It would be unfortunate if someone had to do this because a female visitor looked at your post and thought it was fine to wear a bathing suit in an onsen.

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Short snappy advice from the Tokyo Union of Bathhouses
http://www.1010.or.jp/english/manner.html