If you check the running processes
ps -ef|grep post you can see which version of postgres is currently running. All you have to do is:
1. Stop the postgres version you want to delete / uninstall.
2. Delete the 'bin' and 'data' directories of the postgres version you just stopped.
3. If you are using tablespaces, delete the contents of your tablespace location as well.
4. Remove entries of the postgres version from file '/etc/postgres-reg.ini' and all the postgres version specific files from '/etc/init.d' location.
BTW, if you have used the one step installer to install the postgres, it creates an
uninstall-postgresql file just before the
bin directory location. You can use that to uninstall as well, it will automatically perform above steps.