I did post-graduate work (did not receive a degree) at the same school from which I received my UG degree. Once you actually graduate, no more grades are included in your LSAC GPA calculation, regardless of what sort of institution you attended.
On my academic summary report, there are separate lines for the same school in the "Institutions Attended" (all info came on one single transcript from the school). It is listed with a "G" for post-graduate work, even though some of the courses I took were UG level classes.
Also, I took a course at a community college after graduating, and that too is listed as "G" for graduate work.