I'm looking for ideas to keep people from sitting at my desk
This probably isn't what you want to hear but it's not really your desk. So trying to get them to stop sitting there when you aren't around is ultimately futile and will likely cause conflict and bad feelings.
I attempted to put my chair behind a locked door to have them pull another chair over and was actually annoyed that I had done that.
I'm not surprised that they were annoyed by this, it's an incredibly childish and passive-aggressive response. If you know who the person (or persons) is that use your desk after you leave then you could try talking to them and ask them politely to leave the desk clean after they have used it. Something like:
Hi [Desk-borrowing-coworker], I've got no problem with you using my desk when I'm not around but would you mind taking care to leave it clean afterwards?
If you don't really have the sort of relationship where you can talk to them or if you feel that they wouldn't take it on board then I'd go with Snark Shark's suggestion. Keeping a tub of wet wipes handy and giving it a quick wipe down in a morning might feel annoying when you didn't create the mess in the first place but realistically it's few seconds out of your day at the most and will give you the confidence of knowing that your desk really is clean.
If you don't know who it is or know that it's a variety of people then a polite note on the desk (and perhaps a tub of wet wipes as well) just asking anyone who uses the desk to leave it as they find it is a good way to go. I would say that if it is just one or two people and you do know who they are then a note is a bad idea as, like the locking-the-chair-away tactic it will come across as passive-aggressive and is likely to backfire on you.