What to do when someone is using a personal account name to spam people?

This has happened a few times to me.

I think I am opening an email from a good friend. The address appears to be the same. But when I open it, it is usually for some ridiculous product. The address is often the same email address as a good friend…but it is a bogus email trying to sell me something. Sometimes it is just “gibberish”. Once, I got one from a nephew (who is only ten) and it read: “I drove by your house…. ” and then some random phrases that made no sense. My nephew a) can’t drive and b) lives on the other side of the world and I doubt that his mother would have let him go unaccompanied down the block, much less on a plane across the pond.

How does this happen? How can someone use your name and generate bogus emails from your own account? Is there any way to stop this? (I would like to know to safeguard my own email account from this sort of thing.)

Thanks in advance.