Reviewing papers after leaving research for teaching?
There are (at least) three issues here:


Your time. If you're unable to spend time due to teaching, than that's a perfectly acceptable reason to decline.
Your duty. If you're no longer publishing research, then arguably you're no longer obliged to provide the service of review to those who do. So, don't feel obliged, but you may still enjoy this.
Your expertise may no longer be up to scratch. In particular, you may lack access to some relevant publications and/or may no longer follow recent research to be on top of the field. In this case, you cannot provide an adequate review (w/o an unreasonable effort) and should decline.


In any case, you don't have to feel apologetic when declining a review request, though it does not harm to briefly state a reason.