How do I hide/remove a risk from the matrix and register screens?

The following applies to Risk Register for Jira Data Center and Jira Server

In order to hide the risk from the matrix and register screens, the status must be set to "Closed" (strictly speaking, set to whatever status has ID 6).

The following applies to Risk Register for Jira Cloud

To remove closed risks from the register and matrix, you need to set the status to Closed and also fill in the Resolution field. Some customers modify their workflow to automatically set the Resolution field on the transition to the Closed status.

If you want to use a different status to represent the "closed" state, you can do this by setting a sub-filter on the risk register.