Typically Accept All Changes will remove both redline / blackline markings and comments. However, if that is not doing it for you, then you will want to add the following to your arsenal of tricks.
http://www.microsoft.com/downloads/details.aspx?FamilyId=144E54ED-D43E-42CA-BC7B-5446D34E5360&displaylang=enSome people use different products than Word to markup a document. Some of those other tools just change the text so that it appears as though it is marked up. In which case they have changed the styles. Those documents will need to be cleaned manually - the above tool will not clean them of insertions and deletions. (DeltaView is one of the tools which changes styles)
Dawn Crosier-Bleuel
Word MVP