Is there anyway to block a device that keeps trying to connect to an AP on its own? I have tried the ACL and writing a firewall rule, and while it never makes a connection, it still tries to connect for about 20 seconds every 30 seconds or so. It is a Canon printer, and does not belong to anyone on my network.
Wireless ACL is enough to block connection.
To block the connection attempt you need to either access the settings of the printer or unplug it 
I was hoping there was a way to make it be ignored all together. We are stopping the connection, but it just keeps trying. I think Canon must have bought energizer at some time.
Is this an open network (no SSID password)? or Hotspot?
If so lots of devices tries to connect to it. Like all Apple devices.
Yes it is open, at least for now. Once we get it all working as it is supposed to, a password will be added.
Password as in hotspot web page or SSID password use to connect to the Wifi?
SSID Password, Right now we are just using the ACL.
You can try…
Log into your router’s admin console and change your SSID.
Make your network invisible: In your router settings, choose option: Do not broadcast my SSID.
The above steps will ensure only you are aware of your network. However, it may still be possible to see the traffic using a certain tool which I will not mention.
Change your router password.
Then use mac-address filtering.
Following all the steps above should do the trick of not only denying the freeloaders, access o the internet via your router, but ALSO kicking them off your network for good! As an extra security measure try switching your broadcast frequency to 5GHz which is considerably less used and often requires manual access.