Cut a stream off at some position and save?

If I open an existing file and start writing data, how do I close it at a certain point? Let's say I open a file, compress the data, write the compress data, and then I want to save it. How do I tell Blitz to end the file at a certain position?

You have to delete the old one and create a new one. Or just use something (Asc(0)?) to mark the end of the file.