Thanks for your answer. I found a way to make it work. I changed the restrictions so that only form entries are allowed (except for a specified sectioned paragraph of the document) and with that setting, the user cannot change the formatting of form entries but anything pasted into a form element is formatted the way it should be.
For anybody that finds this thread with the same problem: I am using Office 2007. The problem occured if I pasted anything from outside the document (e.g. from my browser) as well as with stuff from the same document that had different formatting. I had the restrictions set so that no changes were allowed, except for certain exceptions that I set inside the form elements.