Sewer lines need to be vented in order not to create a pressure differential that causes problems. There are a couple different approaches, but that shit isn't one of them; it kind of looks like something a plumber just threw in there to solve their immediate problem and then scooted away from free of consequence.

I wouldn't recommend plugging it, as you might be taking away a needed vent from the whole system... the two options I could see would be:

• Hire a plumber to fix the bullshit in more proper fashion
• Replace that rubber hose at the end with a longer hose that can reach the ground, and put the open end at the bottom of a bucket that you keep filled with water. This is effectively a really weird type of P-trap. It's ghetto but it'll solve the smell problem while still allowing pressure to equalize, I think. You might want to ask your plumber if that would be a sensible solution just to double-check.