To take a song out of your library for good, 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 brought back from the Trash folder until you empty it.

 

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