Windows:
In the pro version, add the following executable files found in this folder: C:\Riot Games\League of Legends\
League of Legends.exe
LeagueClientUxRender.exe
LeagueClientUx.exe
LeagueClient.exe
Only the first file might be required, but a small percentage of users experienced issues with allowances not being counted if the last three files aren't added.
macOS:
Under the "Applications" tab, simply browse to League of Legends.app (and/or Riot Client.app) in /Applications and add it to the block list.