How can I disallow someone from forwarding or replying to an e-mail that I sent?
Do you mean an e-mail that you have sent to them? In other words, you sent someone an e-mail and you want to prevent them from forwarding it to someone else or replying to the e-mail?
You can always eliminate replies by simply enabling incoming message restrictions. Of course, this may have the unintended effect of blocking some or all e-mail, so it may not be what you are looking for in terms of a solution.
As for preventing the forwarding of e-mail, you can't. If you send someone an e-mail message, they will be able to forward it on to others. And even if you could find a way to prevent this (perhaps through some complex programmatic method), you still can't stop them from printing it, taking a screen capture of it while it's on their screen and then printing that, or, of course, reading it aloud.