There are several subsections on the "My RR" tab. Scroll down (to the bottom of the screen) to find the list of audio feeds.
Find the (un)desired feed in the list.
On the far right of the undesired feed's row, click the checkbox in the "Del" column.
Scroll down to the bottom of the list and click the "Update" button.
This might not be the exact step sequence, but it should be close enough to get you started. (Since Broadcastify hasn't fully split from RR, it will redirect you to RR anyway.)