I searched & could not find a way to delete SMS by code.
(i tried smsplus, but maybe it doesn't work with android 4.4?)
so, let me try other approach..
is there a way to group SMS into 1 thread based on contents?
ie: group all SMS containing("received speacial offer") into 1 thread
that way i can still delete manually, by opening standard android Messages & delete-thread.
if that's not possible,
maybe create a contact named:"Spam" and add the multiple numbers that sent me the SMS ..?
hopefully, that would group the spam sms together..
thanx..