Front A frame bolt

That's the front A frame bolt.

If it is just spinning but not coming loose then there is most likely a nut on the top that is supposed to be welded or held in place that has broken loose. You are going to have to access it and hold it with a wrench or vice grips. If it will not break loose, how are you trying to loosen it? Are you using an air impact wrench? If not, you may need one that will break the rust loose and free the bolt. Last resort, you will need a torch and heat the bolt to expand the metal so it can break free.

It has a flat plate with threads.

So its just spinning when you loosen it? Are you using an impact? If so, just heat and then give short fast bursts with the impact to see if it will catch and loosen up. If I am missing something let me know.