My DS2 is of a similar age and completely understands what we are saying to him, so we have now started time out for naughty behaviour. He is asked to stop doing something once, then he is warned that if he doesn't do as told he will be in time out, then it's straight to time out.

He has responded well to this, perhaps you could try something similar?

Just as a side note, with the example you've given above with regard to putting your little one in the car while he hangs on to your leg etc, can you put him in first? I'm sure that would save you a lot of stress and be a lot less dangerous.