Don't bother - it will just make things worse.
If you leave a month before your wedding, they most likely wouldn't go.
On the other hand you might just get a decent present.
If this was only a verbal invitation, you don't have a problem- just don't send them one.
It is absolutely, positively against all rules of etiquette to uninvite someone once they have received an invitation- save for them actually committing a violent crime against you. Just assume they won't come, because it would be hard to imagine they would.
Why aren't they paying you? You can take that up with the Department of Labor.
If these people are as tough as you say they are, I would not challenge them by telling them not to come. There are times it is more dangerous to try and stop something from happening, and poking this snake with a stick could mean they ruin your reputation and ability to ever get another job. The LAST thing you want to do is tell them how awful they are- just tell them this position isn't working out for you, and you are too busy with your upcoming wedding.
I promise you, there is no wording that is going to make a difference. Just assume they won't come, and if for some odd reason they do, smile and be cordial, and move on. It will cost you far less, and if you cannot manage that, you're not mature enough to be married.