The best way to install OpenStack is to install it on a virtual machine and then delete the virtual machine if you no more need it or take snapshots at regular intervals and delete the broken/now-not-needed ones.

If that is not the case you installed OpenStack and now need to completely remove it from your system, do the following on a terminal:

sudo apt-get purge nova* keystone* glance* cinder* horizon* quantum*

and append all services that you installed while installing OpenStack.

After this, to remove the databases, do the following on a terminal:

mysql -u root -p

and then enter your password and then on they mysql prompt

show databases;

and delete all OpenStack databases as:

drop database quantum

and do this for all database related to OpenStack.