On the Standard toolbar, click the drop-down control at the far right of the toolbar.
Choose Add Or Remove Buttons.
From the resulting list, uncheck Reply To All.
You must remove it twice though — from the Standard toolbar in both the main window and the message window. (To access the message window, simply open any message.) Once you remove the Reply To All button, there's no danger that you'll accidentally click it when you meant to click Reply.
If you're thinking that you might need the Reply To All command, don't worry. It's still available:
From the Action menu, choose Reply To All.
Or press [Ctrl]+[Shift]+R.
The process of working through the menu structure to find Reply To All is a good way to ensure that you use that option only when you really mean to.
If you decide you want the button back, simply reverse the above steps using the Add Or Remove Buttons option.
Susan Sales Harkins is an IT consultant, specializing in desktop solutions. Previously, she was editor in chief for The Cobb Group, the world's largest publisher of technical journals.