Has there also been work on the main line? And do you want to forget about that?
Normally I would say: reverse merge everything in the main line to the point where the branch was created. Then merge the branch to the main line. See the section 'Reverse merging' in the documentation for more information. The steps to reverse merge are:
However, you say you do not want to merge. In that case, you could take a checkout of the main line and delete all files except (!) the invisible .svn directory from the project directory. Then copy all files of a checkout of the branch except (!) the .svn directory into the main line checkout directory. Then you can commit the huge change to the main line and this results in the main line being the same as the branch.