Ok, a few things.
1) I'm sorry, but a bowl is totally inappropriate for any fish. I'm not going to be gentle here, the fact of the matter is:
If you keep a fish in a bowl, it WILL die. Period.
Maybe not right away, but slowly, painfully, it will succumb to the massive fluctuations of water parameters and temperature, along with being incredibly cramped.
The solution? Get a legitimate tank. I would get a 5 gallon tank for your betta. You will need a hang on back filter (You MUST have a filter, no exceptions!) also, get a heater, they like their water at ~80 degrees.
Now, for your actual question:
A lot of fishkeepers will be shocked to hear this, but chlorox bleach IS usable to clean a fish tank (NOT when running! When being set up, before being filled!) follow these steps to clean it out.
1) Get a 5-gallon bucket. Fill it halfway with water, halfway with chlorine based bleach.
2) In the case of the rocks, simply dump it over them.
3) Dry them out by placing them in a place where they are spread out (Don't leave them in a bucket! Use a cooking tray or something!)
4) Wait about two days. Then, sift through all the rocks, and smell them very carefully. The human nose can detect bleach at two parts per million. If you can't smell the bleach, it has evaporated and the decorations/ gravel/ tank is now safe for aquarium use. If you can still smell bleach, no big deal, just keep letting it dry until you don't.
So, that's your answer, and PLEASE GET A TANK!
