I've also been plagued by this glitch, but I've been determined to find a solution. Contacting Apple Support was also futile as some have already mentioned. And other "solutions" here have not worked for me nor the community it seems either. So the challenge was afoot and I think I've found a "solution". I've tried this about a dozen times now & every time it has worked perfectly for me. I simply shut off WiFi from the Settings app, not the Control Center & leave it off for about 5 minutes & when I turn it back on, the lockscreen music player widget is gone. I've even watched as the widget simply disappeared from the lockscreen. I'm guessing that there is a timeout limit and since WiFi is off & no connection can be found to an AirPlay device, it simply times out & disappears. I've experimented with shorter time lengths, but 5 minutes or more seems to be the magic number for me. I know this is still not a real solution & it's still a slight nuisance, but, at least for now, I think we have a temporary fix. Please let me know if this "solution" works for you guys as well. Enjoy!

Will do! Thanks!