Is there anything I can tweak so that the AI doesn't build so many SAM infantries when no one has any air units? Thanks

Yes it is possible. It would be done using the CvGameUtils.py.

Incase your not completely python savyy, there are different ways to limit them. You can put a hard limit (say max 25 sam units), or make it depend on several things, like amount of air units on the map, the AI's total population, or their total military. And atleast a hundred different more.

It is also possible (although wierd and kind of gameflowkillish) to create a python script where all SAMs could be converted to another unit on the spot, as well.