I have 3 returns I need to plug. One of them in the stairs I do not have an issue with. For the other two I have plugs with male threads on them. The fitting above is the part that screws into the wall, then the eyeball and outer ring screw onto this.

Any advice how to remove this? I have tried with one of those rubber strap wrenches and had no luck. The amount of threads sticking out of the wall is small, so it is tough to grab a hold of. I would think those 2 indents (you can see 1 in the above pic, the other is on the opposite side) have something to do with removing them maybe. Special tool?

Thanks in advance.