Step children

Y’all. Okay. I have a question.

So I have a step daughter, age 11 & a step son, age 9.

Their mother is absolutely no count at all when it comes to them, sees them maybe 2 hours out of a month but that’s because she’ll be at her mothers bugging for money or stealing from her, when we let the kids go to visit their grandmother. Anywho, here lately we’ve been having trouble with their behavior (as far as smart mouth, throwing things, etc) keep in mind I’ve known these kids since they were 9 & 7, we’ve had full custody of both for a little over 2 years. Well, I try grounding, yelling, taking things away, nothing works. What should I do? I do believe in spanking (I was as a child, please do not come at me with the mom bashing okay if you don’t that’s perfectly fine) but these are not biologically my children, I will however give my own a pat on the butt every so often when I feel it’s needed. Please, what should I do? Is it their age? Most of the time, we all get along perfectly & they have accepted the fact that their mother is no longer in the picture .

L