At the MINIMUM get a plumber in there to tell you what the pipe is for! You could end up with a nightmare if you cut the pipe with no knowledge of its purpose. As stated before, it might be part of the venting (which will affect your other plumbing) or even worse, the actual drain. It would NOT have been installed for no reason in the first place. In the end, it might be easier to make a decorative post around it than to just cut it off and fill it in.