You could do a find and replace for paragraph marks.

In the find dialog box put ^p (paragraph mark) and in the replace dialog box leave it blank. - These can be found under the special command button.

Word of warning - if you choose Replace All it will get rid of all the paragraph marks and you might be left with a one long paragraph.

To avoid this happening, first do a find and replace with: ^p^p in the find box and ^l^l (manual line break) in the replace box.

If this does what you want, then replace the ^l^l with ^p^p when you have got rid of all your underwanted paragraph marks.