As far as I know there is no way to turn off the copy-selection. It's simply the way mIRC copies text. The reason mIRC sends the text when it reaches a linebreak is because it's no different than if you had pressed enter on the keyboard to send the text. If, for some reason, you are continually accidentally pasting multiple lines of text to people, I'd recommend turning on the 'Use multiple line editboxes in chat windows' option located on the mIRC Options dialog (Alt+O) >> Display >> Options.