In the past, if you wanted to remove an RHQ Agent from your RHQ environment, the simple answer was "just uninventory the platform" which would, under the covers, also remove the agent record completely.
However, in some cases, users found it difficult to remove their agent. Usually, what happens is they try to install the RHQ Agent, run into problems, and then get their RHQ system in a state that causes their RHQ Agent to not be able to register with the RHQ Server. For example, this could happen when a person runs the agent as a different user from before or with the -L command line option - both of which essentially purges the agent's security token and could cause the RHQ Server to reject future registration requests from the agent.
If a person does not understand the linkage between platform and agent, or if the agent's platform was never committed to inventory, it became difficult to understand how to get out of the quadmire.
This has now been addressed as an enhancement as requested by BZ 849711. Now, the answer is simple - regardless of whether the platform is in inventory or not, and even if the agent's resources do not yet show up in the discovery queue - you have a way to quickly purge your agent from the system. This will allow you to get back to a clean slate and attempt to re-install your agent.
You do this by going to the top Adminstration page and selecting the "Agents" item. From here, you see the list of all the agents currently registered in your RHQ environment. If you select one or more of them, you now have the option of pressing the new "Delete" button at the bottom. This will do a few things. First, if the agent's platform is already in inventory, it will uninventory that platform. This means that platform and all its child servers and services will be removed (so be careful and make sure you really want to do this - you will lose all manageability and all audit history for all resources previously managed by that agent). Once that is done, all resources will disappear from the inventory and you won't even see any resources for that agent in the Discovery Queue. Finally, the agent's record itself is removed - so the Administration>Agents page will show that the agents have disappeared.
With the agent and its resources completely removed, you have the option to attempt to re-install the agent if you wish to bring it back.
You can also use this feature if your managed infrastructure has changed and you no longer want to manage a machine. Just select the agent that was responsible for managing that machine and delete it.
Note that if your agent is still running, it will attempt to re-register itself! So if you no longer wish to manage a machine, make sure you shutdown the agent as well (you'll obviously want to do this anyway, since you won't want an RHQ Agent consuming resources on a machine that you no longer want managed by RHQ).