warning: refname 'HEAD' is ambiguous

Also, this will delete the branch, if you just don't want it.

git branch -d HEAD

Use a capital -D to force the deletion:

git branch -D HEAD

The problem is that you have a branch called HEAD which is absolutely dangerous, since that's the symbolic name for whatever branch is the current branch.

Rename it:

git branch -m HEAD newbranch

then you can examine it and decide what to do (delete it, or save under a descriptive branch name)

(The origin/HEAD remote branch is not a problem)

If you have created a tag named HEAD using...

git tag HEAD

...you can just delete that tag using:

git tag -d HEAD

See this case: kerneltrap.org/git-tag HEAD