So do what Peter said: Just clear all the contents of the text box and presto. You will be giving the user a blank field, again. Yes, with the cursor at the start.
As for the ending NEWLINE character: Two ways.
(2) Using C#, just wipe out the last one or two characters of the string. (You'll need to check to see if the ENTER key put just "\n" on the end of the text [Unix style] or "\r\n" [Windows style].)
Lots of ways to do that using the various string class functions. Just look up the .NET docs for the String class.
Huh. I didn't even know this method existed! Look how easy they have made it for you: