people who cant ride a damn bike right

I work at a bike rental shop and I see so many people who cant ride a bike. People get on a bike and crash into cars and kids and shit because they cant control the handle bars. tThey don't know how to shift gears and complain that they cant ride the bike because its in the highest possible setting. something els that bothers me is when I people riding around with there seat way to low or the teenagers that ride a bike that is built for a 10 year old all that is going to do is make them tired extremely fast.