Darn those plastic tools and other toys that can be used as hammers, saws, or worse! When our boys were young, each one had his phase where they damaged just about every piece of fine wood furniture we have by leaving unsightly marks and marring, and by pounding dents in them with their tools. However, remember, boys will be boys, and it's in their genes to "fix" things, to "pound" things, and to "see what happens if". The good news is this is a phase, and has nothing to do with any failure or mistake on your part. This is an experience thing that only a mother could acquire. Be patient and wise.
I was a little slow on the get go, but finally figured out the solution is as simple as banning any toys that can be used to damage things. Don't allow anyone to purchase them as gifts, and if you must appease the little Bob the Builder in your house, invest in foam tools, which you can purchase from distributors that cater to preschools and pediatricians. They have a wide array of soft toys that appeal to the tiny builder in your life. One place to check out is www.discountschoolsupply.com.
While this may seem unfair or limiting, you will find it is worth it, since often the children who find enjoyment in being somewhat destructive are also the ones who may experiment with doing these things to another child is just as fun. Before it gets to this point, you need to reinforce that it's important to respect property and if necessary other people. They also need to be taught how to find alternative activities that give them same gratification and learning experience (hard to believe they're learning something, but they are, these are the hands on learners who get alot out of cause and effect activities. My boys, now older are both very technically minded and enjoy complicated building sets etc.)
So back to what to do now, the minute we saw them trying to use something like a "tool" we'd say "No!" and take the toy away from them indefinitely. We would first give them a chance to prove they won't misuse a toy by first reinforcing that these toys could only be used to pound on other "workshop" type toys such as their plastic tool bench, or special toy cars made for tools. We would further explain that if they used the tools or any toy in a way that it was not meant to be used, it would go away permanently. After sticking to this, they started to shape up tremendously. As I mentioned earlier, I think finding toys that fulfill that need to explore in a technical yet hands on way is a great way to distract them.
If the damage they caused was bad enough they would be punished with time outs and small chores to rectify the damage they caused such as help dad clean the walls or furniture they marked up etc.
Bottom line patience and creativity. They will come around. Just remember he's probably really smart and needs that intelligence focused.