You actually listen to the HR people? Well there's your first mistake. They DON'T care about you or your life. Second, you signed something you didn't hardly understand. If something is soooo important that you have to sign it, ask the HR bitch what you're signing.
As far as wage garnishing, technically the answer is yes but in reality its no. They would have to sue and that would cost a lot more than the union dues, so they'll give it up if you get fired or something.
Don't ever sign anything you don't understand and don't ever let anyone tell you that you have to. You never have to do anything someone tells you in life. Now that you have signed it you can't do anything; it's legally binding now. The only option you have left is if you do start noticing more money missing from your check and you can find out that it is because of this you have 1 last option. Save your pay stubs, if an extra amount is coming out the stub must say what it is for and how much. If it doe's not, someone (not you) is in trouble. Also, you could take Kroger to small claims court and sue for the money that has been taken from you but this also cost money to do. And I am willing to bet, the amount that may or may not be taken from your check for these past union dues, will not be enough money for you to spend more money and time taking Kroger to court. If you have to take this rout, you'r only option if you signed under duress with the believe you'r job was in jeopardy if you didn't. Make sure you find out how much should be coming out of your check. you don't want this to be an ongoing thing.
if they ask you to sign anything insist on a copy for yourself. ask questions.
look at your past pay stubs and see if the union's been taking dues. did you join the union before? you think you did not.
you need to find out about this. the union would have a record of you joining. there's a form to fill out and usually it goes to one of the union stewards in your store to pass along to the union rep.
you can look at a prior pay stub and it will tell you how much in union dues you paid for the current year.