How can I stop someone from sending spam using my domain?

That happened to me, too. Contact your ISP or if you can change it yourself, change all domain access passwords and all email access passwords especially the generic one [mydomain@mydomain.com], increase your email spam protection,

With some ISPs you can configure the settings for SpamAssassin yourself

Also if you get NOWHERE with your ISP consider moving your site to another provider.

for a while I collected some of the really odd incoming emails I was getting—which seemed like notifications, maybe green-light signals, for spammers to go ahead. very odd short coded things just a couple of gibberish letters in the subject and a few more in the body.

I just found the Q I posted when I had this problem. It even has screen grab samples of the spam.
http://www.fluther.com/118013/does-this-look-like-slave-computers-making-spam-were-run-through/

I was going to do something recommended at the end of the previous thread – use gmail’s “show original” to see all the header code, both for the emails with the letters and numbers and for the emails from people at my address, but alas, too late, I got rid of all that stuff a year ago and set up my SpamAssassin. I don’t think I even saved any of those email from myself. If I find any I will show them . . .