How do I remove closed risks from the register and matrix?

This applies to the Cloud version of Risk Register

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.