His step dad

You can elope and not invite him or invite him and have some people forewarned to run interference if he starts to cause drama.

If you mean opinions, like on what foods to have and such, listen as you would to anyone in any group, whom you may not like, but are civil too.
Or say, you and FI had planned something different, you will see.And change the subject.
You don't have to agree or make any changes. And even annoying people you don't like to spend time with, sometimes have a good idea.

Putting in his two cents isn't really a big deal because you can easily ignore it. You don't have to listen to his input. Just politely say "thank you for that, we'll keep it in mind" and then promptly forget it.

View Quoted Comment I totally agree with Elizabeth. Smile, nod, and ignore it. Most of us have someone offering their unsolicited and unwanted opinions for our big days and it’s totally inconsequential. Be civil, but you don’t have to change a thing.

I’m in a similar situation. My fiancé’s father’s wife hates me, so we had to decide what to do as well. We believe he might not come if she doesn’t, so we decided to invite both.
I see it this way, how much time will actually be spent talking to or around that person on your wedding day? And it’s better to invite them, so they can never turn it around and say you didn’t.
If your fiancé’s stepdad tries to add his opinion into your wedding, just say we thank you for your input, and leave it at that. It’s your wedding! Especially if you and your fiancé are paying for the wedding, no one else decided anything except you two! I hope it works out!

Just like anything else people press their unwanted opinions on, just smile and say non-committed responses like:
Thank you for sharing your opinion.
That is certainly something to think about.We haven’t decided on that yet.We’re not sharing all the details of the wedding because we want some things to be a surprise. (That one is my personal favorite because it’s soooo non-committing!)

View Quoted Comment My fiancé doesn’t like his step dad so I could potentially just speak my mind but I don’t want his mom not to be there because I flipping love her

Remember the phrase/question “If you weren’t related to them, would you have anything to do with them?” And act accordingly.