In the contact sensor settings, you are able to get access to more information, move it from one room to another, or delete it.

You can access those options by opening the drop down next to the product name on its settings page.

If you wish to delete the contact sensor from your home, you will be prompted with a confirmation tab as all events associated with this product will also get deleted.
Then, the contact sensor being only updated from time to time from the cloud, you can either wait up to 24 hours, and it will receive its deletion message, or press the Setup button inside the product on the back  to reset it and forget all network information that may be in memory of the contact sensor.