Use 'reset()' in a script

edit

This is something you can pretty much try for yourself.

$ cat test.sage 
A = matrix([[1,2,3],[4,5,6]])
print A
reset()
print A
$ sage test.sage 
[1 2 3]
[4 5 6]
Traceback (most recent call last):
  File "test.py", line 7, in <module>
    print A
NameError: name 'A' is not defined

So I guess it works.