One of those simple ones that is always good to think about ahead of time.
Step 1: Put a nut or two onto the bolt/rod and tighten down past the point that will be cut off
Step 2: Cut bolt/rod with hacksaw, grinder, or bolt cutters
Step 3: File lightly to remove any flanging caused by cutting the metal
Step 4: Apply a few drops of oil (3-in-1, etc) and then slowly remove each nut, using a wrench if necessary.
Rock the nut back and forth if binding is encountered to allow the threads on the nut to re-cut the threads on the bolt.
Quick, simple and cheap.