If you want to work from the start just drop your RHQ database, server and agents and reinstall from scratch. Beware that agents store config in preferences file located in the home folder of the user which ran them. When starting your agents again use the --clean argument to wipe those files.
Many tips on how to use RHQ in the project wiki: https://docs.jboss.org/author/display/RHQ/Home
Thank you so much for your guidelines. I will be very thankful for this.