I'm inclined to agree about removing the Paste button by default and leaving only the Paste as Text and Paste from Word buttons, I may do that for the next release. You can configure it yourself, though, see the documentation for customizing the editor toolbars. It does require some knowledge of the specific editor you are using.
I'm not sure about a way to force Paste From Word, it is possible to force Paste As Text, in fact I have that configured for the forums because people always scrape html and paste it in. I'm not sure I have exposed a property for that in the TinyMCE configuration (will look into that for the next release of mojoPortal), but using FCKeditor you can set this true in a custom config file:
FCKConfig.ForcePasteAsPlainText = false ;
Hope it helps,