I'm against spanking too, but if and when children need discipline (and they sometimes do), there are effective ways to do it that don't involve assault.

Screaming at a child and/or making empty threats only makes an ass of the parent.  So does trying to reason with a 4 year old as if he or she is actually 40. Children NEED to know that an adult is in charge and that they are EXPECTED to behave in a respectful fashion.

An example of an alternative to spanking:   Banning a favorite activity (i.e. video games) for a designated period of time. 

1/ Model the type of behavior you expect from your children. (Children don't respect hypocrisy, nor should they.) 2/ Be fair (but do not negotiate). 3/ If and when you must discipline, say what you mean and mean what you say. 

Random example: If you catch Little Schnookums intentionally damaging the furniture, do what it takes in terms of discipline to ensure the child does not do that again in future -  AND DON'T BACK DOWN.  

Be a parent.