I had hoped that someone else would point this out because I've not used the feature, but I know that you can, in your template, restrict editing to a given set of styles.
00 deleteme 3.png

The control for this is found on either the Developer Tab or Review Tab. To protect against importing a style definition for a style that is allowed, the style must be actually already used in your template.

As Graham pointed out, the best solution is for your users to paste as plain text.

You would want to protect the document for use of designated styles only, using a password that you write down.