Quite so. I have blanket-banned myself from sponsoring people for three years now. My co-workers know that there is an exclusion zone around my desk within which no charity donation will be made.
I'm not against raising money for charity, but as many have said I don't think what most people do is in any way remarkable and most of them couldn't tell me the first thing about the charity they're supposedly helping.
I realise that running 5 km might be a big effort for someone, but that's their fault, not mine, the lazy bar stewards.