"Resetting" a repository

I have a project with a test repository and a live repository (and a 
development repository).

Over time, the test repository has had merges which were in a quite 
different order to the live repository, so it no longer fulfills its 
role of being pretty much like the live repository and therefore a good 
place to test. The two have diverged too far.

We've reached a point in the project when it would be good to reset the 
test repository so that it is exactly the same as the live one. We can 
then merge new work from the development repository.

I think that if I just merge the live repository into the test 
respository it will not remove extra (non-conflicting) code. On this 
occasion I want to remove all extra code and resolve all conflicts in 
favour of the live repository version.

I'd appreciate advice on the best way to do this.

Thanks

Roddie Grant