mrXmr Composer is usually a single executable file, if you know where it is you can delete it with root privilege.
If you don't know where it is but access it via
composer, you can run
which composer to find its location (for example, on my system it returns
If you uninstalled Composer then you can also delete the entire
~/.composer folder that holds any Composer configuration or cache. There will be one hidden Composer folder for every shell user that ever ran Composer.