Is the finish oil based or water based?

I would ask the flooring person how to remove it, or ask that they take care of it for you.

But knowing exactly what product was used will help. Mineral spirits or something like that might take it off pretty easily, although I would be nervous about discoloring or damaging the composite.

The people who sold us the house finished the floors for us before we moved in. I'm guessing oil based as it had to seep in and dry several days before we moved in.

I would find out what materials can be used on the composite deck. And while you are at it, you might want to find out if you can get 1-2 replacement planks in the same colour as the deck...just in case.

Once you have a list of materials you can use on the composite deck AND you KNOW you can replace planks with new material in the same colour, you can go ahead and attack the foot prints.

You will need to find the chemical that dissolves the FLOOR FINISH without dissolving your deck. I'm going to guess you will need to scrub AND sand this down. The question becomes: When to STOP and simply have the planks replaced?

It would probably be easier/faster to replace the planks than it would be clean them. But it will cost more money to do so.

Find out if you can access replacement planks for your deck before you do anything. That would be where I would start. If you can't, then be VERY careful with what you use on the deck. The cure may be worse than the disease.