For most toddlers no is meaningless...as is stop. Stop telling the child what NOT to do and start telling the child WHAT to do. "No" should be used on a limited basis. Hands down. hands to your self, find another toy, back up and give so-and-so space, sit down, stand up etc are all better messages as it gives the child info as to what you want them to do. Doesn't mean they will get it right away or respond how you want them to (they are toddlers) but kids are very very quick to tune out the word 'no'.

If the child is constantly waling into things and fall over etc...suggest the parents have vision and ears checked out to rule out any larger problem.