That "flange" is the driver assembly that he was referring to. I use a penetrating oil called "KROIL". I buy it from Amazon. It works fast to eat through rust and crud. We all do things differently, but I would get a piece of rubber, like old tire, lay that on the engine side cover and gentle pry against the back of that driver. Turn the crankshaft and pry again, tapping on the end of the sleeve in the center as I pry. Try not to bend anything.
It's been on there for a long time and it might take a while to get it off. Once you get that off(It's the sleeve in the middle and the big flat disc) You will see the seal that Markus told you how to remove. Good luck.
EDIT make sure there is not a bolt in the center of that sleeve, screwed into the crankshaft.