It's not always clear how to delete a remote branch in git, but it's just the following command:

git push REMOTENAME :BRANCHNAME

 The syntax is a little wonky if you're not used to the advanced push syntax. If you look at the details of what's happening you're telling git to push nothing into the remote branch.

git push REMOTENAME LOCALBRANCHNAME:REMOTEBRANCHNAME

I'm not usually on a Mac, but several people I work with are and periodically they need to flush their local cache. I'm just throwing this here because I keep having to look this up. Just open the Terminal up and type the following:

dscacheutil -flushcache