Do they? I thought this kind of thing - stones etc - ended as was replaced with regular heckling. It's only Mpeketoni I see stones being thrown.
About Joho saying something: Do you know of what he said to grant the Mpeketoni the excuse to throw stones at him? I assumed they are unhappy because he is opposing Uthamaki.
Kenyans of all tribes mistreat political opponents, even those from the same tribe - it's a national culture. Did Joho say something to upset the Kikuyu in particular?
kamwana was once attacked with shoes in Migori as he tried to launch an anti-Malaria campaign. This kind of thing happens, though usually under more explicable circumstances than an anti-Malaria drive.
The reason you think heckling is the limit, is also the reason I am thinking Joho must have said something, I don't know what. It could also be that those folks are just attacking him because he is not Kiuk, which I find hard to believe.
None of what I am saying, is an attempt to excuse the behavior.