1. a youtube to MP3 converter (i prefer youtube-mp3.org)
2.your new vegas folder (C:\Program Files (x86)\Steam\steamapps\common\Fallout New Vegas)
3.an intelligence of 5 or more
4.a backup save in case something ♥♥♥♥♥ up horribly (thanks Haxxi)
5.an audio converter

1. Save corruption (untested, possibly only happens if you save while the song you want to replace is playing, and then replacing it)
2.infinite loop (might also happen when you do the above)
3.song repeats itself and extends past the regular song (this WILL happen)

Here's what you need to do to remove Johnny Guitar
1. go into your new vegas folder (C:\Program Files (x86)\Steam\steamapps\common\Fallout New Vegas)
2. go into Data
3. go into the Sounds folder
4. go into songs
5. go into radionv
6. find MUS_Johnny_Guitar
this is where the downloaded MP3 comes in. remember that downloaded audio converter I mentioned at the start? change the MP3 to a WAV file. For whatever reason, new vegas hates MP3s, so a WAV is your best bet for the least buggy experience (unless it's actually worse, in that case, use the MP3). you need to place it in the folder, and move johnny guitar elsewhere or delete it. Rename the converted WAV to MUS_Johnny_Guitar, and exit the folder. Start new vegas up and check the radio stations to see if the new song plays.
Enjoy your Johnny Guitar free wasteland!
This also works on other songs, not just Johnny Guitar.