Perhaps a cheap pair of bolt cutters to size them down? Sandpaper to finish, if needed.
That doesn't seem like a good idea to me--too much chance of damaging threads beyond repair, or actually flattening the bolts.If you can borrow or buy one easily, threading one of these, https://www.amazon.com/Irwin-9427-Hanson-18-Thread/dp/B00E9J9JBO, over the bolt prior to the cut makes it easy to clean up any damage.