To remove an idea from your active ideas list (backlog) you can archive it. 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.
To archive an idea:
Within the Roadmap
Find the idea on the roadmap, click on the state and switch it to Archive
Within the idea canvas
Click on the state and switch it to Archive