I adopted a 7 year old dog aggressive female Staffy Bull/ Jack Russell mix. Is there anything I can do about her reaction to other dogs at her older age?

Hello,

I have recently rescued a 7 year old Staffordshire Bull terrior/Jack Russell, her name is Penny. She is well behaved and has understood from the off that I am master. I was told she was dog aggressive and I am confident enough and usually able to distract her we she sees other dogs whilst out on the leash. However, yesterday as I walked round a blind corner another domesticated dog came with it's owner, Penny instantly lunged and bit the dog around the snout before I could react, thankfully I pulled her off before she could really bite down. I pinned her head to the floor and shouted at her I then took her straight back in the house and regrettably gave her a firm smack on the back end then put her outside for the day. I don't know if I reacted correctly or how I can prevent this in the future. I'm assuming by the severity of her reaction to just seeing the dog in close proximity and her older age there isn't much I can do?

Thanks for any advice you could give.

Kind regards,
Glen