The only difficulty is that TeX Live is owned by root and lives in a location that is not commonly visible in the Finder. But it is easy to work around this problem. In the Finder's Go menu, select "Go to Folder"; in the resulting dialog window, type "/usr/local/texlive". You will see folders containing various copies of TeX Live. Drag the folder corresponding to the version of TeX Live you want to uninstall to the trash. You will need to provide an administrative password when asked.
The folder texmf-local is available for local additions to TeX. These local additions are used by all versions of TeX Live, so if you added files for TeX Live 2014, they are still around for TeX Live 2015. When MacTeX is installed, it doesn't touch texmf-local if it already exists, but it creates an empty directory tree if none is present.
Consequently, if you are updating TeX Live to a new version, you want to leave texmf-local alone. But if you are trying to remove all traces of TeX, then remove texmf-local by dragging it to the trash.
