How do I lock Revit families?

If you are in a work sharing environment, don't lock or pin anything with the possible exception of the structural grid. Later in the job, users working on CD development will waste more time on unlocking than on productive work. My experience is that users rarely think through their locking schemes. I have found work that was plainly wrong that had to be fixed, but was locked up like Fort Knox. And remember, the time you spend locking things could be better spent on project development.

no, you cannot lock families

The question is why do you want it lock? If you don't want people to tweak it the problem is many people would probably not download it.

You can't lock files but you can make it harder to change files I guess.

Sorry, but you actually can't literally lock files.

You can't really prevent a end user from going into the family and editing it, but what you can do so an end user cant manipulate the parameters is to lock the out with formulas for the parameters with the value listed
there's also more complex way to control the dimensions to prevent the end user from going below minimums and maximums with a CTRL Parameter with the formula like: if(Width<1'2",1'2",if(Width>4'8",4'8",Width))