To take a song out of your library permanently, move it to the Trash. Tap or click Songs to enter the main library, then swipe a song (right-click on Mac and Windows) and choose Trash.

 

Trashing a song also removes it from every playlist it was in. It can be restored from the Trash until you empty it.

 

For more, see Organize your songs into playlists on the iReal Pro website.