Mail rule to bounce mail

Actually, you shouldn't be bouncing spam manually or otherwise.

The vast majority of spam is not received from a valid sending or return email address. If the email address is valid, this is likely due to a spammer taking over a Windows PC remotely, or using a valid email address from the spammer's "known good" email address list to be used as the sending email address for their garbage, which is changed from among their huge list of valid email addresses.

The best thing you can do is mark the message as spam and be done with it.

Don't reply to or bounce the message, don't render any HTML or any embedded images or objects included with the message, which confirms that your email address is valid causing even more spam to be received, don't select any links included with the message which will also confirm that your email address is valid.

Once an email address is severely compromised and the cat is out of the bag, you usually don't have any other choice but to use a different email address, unless your spam filter is working well.

The above point about spam is well taken, but what about making a Mail rule to bounce back emails from specific non-spam email addresses? Is there a way to do this, perhaps using Applescript?

There is no rule selection to Bounce a message received from a designated email address, but you can select to run an available AppleScript. I assume there is a way to write an AppleScript that will bounce the message.

Try this, although I haven't tried to test it at all:

using terms from application "Mail"
 on perform mail action with messages theMessages for rule theRule
 tell application "Mail"
 repeat with eachMessage in theMessages
 try
 bounce eachMessage
 end try
 end repeat
 end tell
 end perform mail action with messages
end using terms from