To do this, from a root shell:
# /etc/init.d/groundwork stop gwservices
then from a root mysql cli:
drop database jbossportal;
create database if not exists jbossportal;
grant all on jbossportal.* to 'jboss'@'localhost' identified by 'jboss';
than back at root shell:
# /etc/init.d/groundwork start gwservices
This will wipe out all non-default users and passwords, as well as any custom portal pages, dashboards, etc.
It's a good idea to back up the jbossportal database before customizing dashboards to avoid this problem.