Fanya ivi, next time you hear or suspect wanacheka go up to them and start a playful conversation juu ya Kenye wanacheka... Hio story itaisha hapo. Part of why they are doing that is because the subject is semi taboo hence funny. Everybody has wanked in one way or another. I personally don't do it anymore but don't care if someone does it, furthermore kwa nyumba unalipia. if you make it seem like a small thing to you, it won't be anything more to them