How can I explicitly free memory in Python?

I wrote a Python program that acts on a large input file to create a few million objects representing triangles. The algorithm is:

1. read an input file
2. process the file and create a list of triangles, represented by their vertices
3. output the vertices in the OFF format: a list of vertices followed by a list of triangles. The triangles are represented by indices into the list of vertices

The requirement of OFF that I print out the complete list of vertices before I print out the triangles means that I have to hold the list of triangles in memory before I write the output to file. In the meanwhile I'm getting memory errors because of the sizes of the lists.

What is the best way to tell Python that I no longer need some of the data, and it can be freed?