You can uninstall or removes an installed aria2 package itself from Ubuntu 12.04 LTS (Precise Pangolin) through the terminal,
$ sudo apt-get remove aria2
If you would like to remove aria2 and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove aria2
If you use with purge options to aria2 package all the configuration and dependent packages will be removed.
$ sudo apt-get purge aria2
If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.
$ sudo apt-get purge --auto-remove aria2