I am planning on replacing my wood deck boards with azek..some of my deck screw are rusted and stripped.can i use a reciprocating saw with a metal blade to cut between my deck boards and joists to cut the screws and joist level?will this affect the screws going into the joist when i lay my azek and screw to joist using screw going into the sides of the azek?
You can do that, but some screws are very hard and you'll have a slow, noisy time of it. You'd want a high quality metal-cutting blade. Getting a gap between the decking and joists makes it much easier, too.
If you aren't trying to salvage the decking, just pry the boards off the joists and then bend the screws that remain back and forth a few times. They'll snap off fairly easily, and you can pound any sharp stubs flat. You could make some circular saw cuts through the decking between joists to help you pry things loose.