I actually prefer that you can click on the text next to the checkbox to select it. This is true with radio buttons too.
If you really need to prevent it, unlink the label from the checkbox or radio button. And check the size of the checkbox control. Shrink it to 20x20.
FWIW - the bigest UI issue my users have had was when a drop-down list covers other controls. And users would double click the item in the list. The first click would select the drop-down item, the second would toggle a checkbox that was under the drop-down list when it retracted.