To remove an idea from your Backlog, you can simply move it to the Archive tab. This places it in a suspended state and does not delete any information, so you can always make the idea active again when it is ready to be worked on at a later time.
Within the Roadmap
Find the idea on the roadmap, click on the state and switch it to Archive
Within an idea
Click on "Move to..." at the top right of the idea page, and select 'Archived.'