Banning VPN use wouldn't help on its own, A DDOS attack uses other peoples computers to send fraudulent requests for service to the server repeatedly. A temporary fix could be limiting the amount of times a certain IP address can send a new request to join in a certain timeframe in addition to banning VPN use, with that timeframe being as long as possible while impacting the fewest players actually trying to play (maybe like 15 times in a 24 hour period) with a captcha to bypass it for legitimate players who just had to relaunch multiple times because of issues.