Sewer smell from basement drain with no p-trap. Can I plug/seal it?

[Lightthief]

I have a 4in open drain in the basement.
It's been bone dry for the year I've owned this home and not been a problem.
Lately sewer gas smell has been coming from the drain though.
The drain has no trap, I've run tons of water through it and it just drains straight out.

I don't need the drain, my guess is that it used to be a utility sink that was removed, as I have a sump pump and a p-trapped drain for the washer.

Can I plug this?
If so what is my best option for plugging it?
Is gas build up a potential problem?

Would appreciate any advice.

 

[frodo]

stuff a towel in the pipe, about 4'' down
mix up a bucket of concrete and pour the hole