Check in the contacts panel and make sure you don't have any contacts that have multiple email addresses set in their properties. Right-click oh a suspect contact, got properties and make sure there are no additional email addresses listed on the notes tab. A bug can cause mailing lists to be created based on those additional email addresses. Once you fix/delete any problem contacts, delete those mailing lists again and they shouldn't come back. For any contacts that you want to have multiple addresses, just create separate contacts for each address. You can right-click in the contacts panel and create a folder to group them if you want.
If you're not running into that bug, then you're just encountering emails with a List-Id header, which tells Opera that the message is from a mailing list. You can right-click a message and choose "view all headers and message" to check for the header. Maybe you can find the problem messages and block them from coming in again via server-side filtering.
When Opera detects a mailing list message, Opera automatically creates a mailing list view under "Mailing Lists" if the view for that list isn't already there. There's no option to turn that off.
What you can do is right-click in the "Mailing Lists" section and create a folder. Name it "hidden". Then, instead of deleting all those problem mailing lists, drag them into the "hidden" folder and collapse the folder so it doesn't take up as much vertical space.
Its pretty simple really. Opera needs to make money to keep going and to do that it has to have enough market share for companies to what to give it money to add their search engine, bookmarks and such to its browser. Meanwhile its market share has been stuck in the single digits for 20 years. They tried playing the game by being the stand out that had tons of features built in and their own custom rendering engine and it didn't get them anywhere. Meanwhile the rest of the industry went for clean, simple, basic browsers and, outside of IE, they were released after Opera and quickly passed it in market share. Firefox, Safari, Chrome, all came out years after Opera and beat it by catering to average users and mostly ignoring power users.
Now look at Opera's position; it has an aging custom renderer which was becoming a problem to keep current, an interface so integrated with the renderer that it couldn't easily be swapped out, flat user growth and a mobile browser that went the clean, simple, basic route and is one of the heavy hitters in the mobile world. They saw what way the wind was blowing and they changed their tack to follow it.
Vivaldi is targeting power users and is trying to be the replacement for Opera 12 but I doubt they will ever have more than a single digit market share. Not that that is a bad thing. If you can carve out a niche and make it work more power to them. I have it installed myself. I will be keeping an eye on it to see if it becomes something worth using. The thing you have to understand is, at the end of the day, making software is a business and its hard to keep the lights on if you aren't bringing in any money. Opera was tired of being stuck in a niche in the desktop world and they saw they had to make some hard choices if they wanted to carve out a bigger piece. They decided to make the changes necessary to do that instead of trying to keep their old renderer banging along. Yeah, it pissed a lot of people off, but as Woodrow Wilson once said: "If you want to make enemies, try to change something."