Take a stand...
Hitting is never right, and should NEVER be accepted. you have to make it clear in no uncertain terms to your children that hitting shall not, and will not ever be tolerated. if you do not make it clear right way, you will have kids that grow up to hit wives etc and maybe get into fights etc. Any decent psychologist will tell you this, and if you do not know how to stop it without screaming, there are classes for parents that need help.
Tie outs, with stern talk should do the job...and with unruly children who refuse to listen, you need to show those type children that you will outlast them, no matter how much patience that takes...an you will win no matter what.
When you become a parent you take on an awesome responsibility to be willing to do whatever is right, no matter how much that demands from you, so be willing to put in a tremendous amount of effort to assure that your children know right from wrong, and the consequences...right from day one. The longer you wait to do this, the harder it will be to break them of horrible habits that can lead to terrible consequences like jail and loss of loved ones later.
The time to address this is NOW.