If you're looking to do this board wide or even per-forum, here's how:
Go to the permissions tab in the admin panel and select the proper forum(s) and user(s) or group(s) that you want to change. Hit edit permissions. It should pop up a screen with simple pulldown boxes of Full Access, Limited Access, etc...
To the right will be an "Advanced Permissions" link, click that then choose the "Actions" tab from the next screen and you should see the options to change "Can delete own posts" and "Can edit own posts".
If you want to do it board-wide, simply change the Role mask.
Go Into Permissions -> forum roles.
Click the green-gear edit icon next to Standard Access, or Standard Access+Polls, whichever one you are using for standard users.
On the Actions tab, set "Can delete own posts" and "Can edit own posts" to "No"