A redirect type of mod based on user agent should be do-able (just not by me today too busy) but until you have time to dig into it deeper or find someone with time to code it you can try:
https://vborg.vbsupport.ru/showthread.php?t=268208
Doesn't matter if they're a spider/bot or not it will ban the user agent, the thing about Opera Mini you must realize is the fact it has so many user agents, so one of the older mods/plugins you used prior may have worked HOWEVER may not of contained references to all the OM agent strings therefor it wasn't always detected etc.
Reference:
http://www.useragentstring.com/pages...ame=Opera+Mini