No, it's not a good idea to slap a child, regardless of their age. You can help a child learn by telling the child 'No' when he reaches for something, by removing the child from the area and by removing the object. As children become more mobile by creeping and crawling, and eventually by climbing and walking, they explore their surroundings and express their natural curiosity. They have no notion about what is safe and what is not; they graduakky learn this by interacting with their physical and social worlds. It's important for parents to be vigilant and remove objects from reach in order to protect young children, at the same time we teach them by talking with them.
No! But, a simple change in inflection in your voice could be effective. Don't yell, but simply say "no" in a firm voice - not an angry one. But be sure to give hugs and loving communication when they're doing nothing at all - even just sitting there looking cute, which is 99% of the time. If an item is in their reach that could harm them, which is just about anything at this age, please remove it and put it out of reach. A child of nine months should explore - that's how they learn. Curiosity is good. Allow things to be in reach that is good for them - toys that can't be swallowed, but are fun to look at and hold. Your house may not look the same for awhile, but the safety of your child doesn't even compare. Be sure to get good electrical plug covers, preferably the ones with a spring action closure. Also, lamps where the cord can be grabbed and the lamp yanked off the table should be out of reach. Prevention of accidents is key here. Also, if you don't already, get good cabinet safety hardware that prevents the child from opening cabinets and drawers and smashing those little fingers. Last bit of advice, be sure your oven/stove has an anti-tip bracket installed. If your bundle of curiosity should try to climb up on the stove to see what's up there, they could tip the whole unit over on themselves. I'm not assuming you haven't taken any precautions, I'm just reinforcing what all parents need to do as sometimes we can get too busy to take the time to be safe. We ALL need reminders, including myself.
Hindsight is 20/20.
Sincerely,
Mother of four kiddos.