Sometimes after starting a reply with a quote I think better of what I want to say, so I just want the reply I’ve started to go away. Is there a simple way to do that? The only thing I have been able figure out is to close the app. Reopen it then select clear the text box after it gives me the option. Is there a way to cancel the reply post without leaving the forum?

Since you mention closing an app I presume you're using some kind of a forum app for your phone?  I use a regular browser (safari in my case) when using the forum and I find that the forum engine cooperates nicely with my iphone browser, I used to use dedicated forum apps but didn't like them, try using regular Google Chrome for your phone.