Updated Answer
As @user3159253 stated in comments of this answer :
git garbage-collects commits which aren’t referenced, directly or indirectly, by a named reference (branch, tag, etc). That is why it is important to leave a reference to a freezed branch.
You can tag the tip of the branch by archiving it, and then delete the branch.
git tag archive/<branchname> <branchname>
git branch -d <branchname>
git checkout master
The branch will be deleted, and can be retrieved later by checking out the tag, and recreating the branch.
git checkout archive/<branchname>
git checkout -b new_branch_name
Or more simply :
git checkout -b new_branch_name archive/<branchname>