No way to prevent it unless they sort it out themselves. Chickens bleed real easy, it doesn't take much of a fight to make it look serious. The question is how long are they fighting for, and how soon do they return to it? If it's chronic and consistent, they have not yet sorted out their differences. Sometimes they will, other times they won't.
If they keep at it, you'll need to pen them up into their own "territory", or keep one penned while the other forages and rotate days, or find a home for one.
It can be dangerous for either bird if you let them sort it out, entire combs ripped off or other damage to the eyes, waddles, etc. Or just consistent scabs being on the comb from small confrontations. It just depends on the boys involved.
Be aware they can really beat themselves up between a fence as well. If you crate one for a day at ground level, the other may still instigate a fight through the cage wire. You'd want to block off any shared walls, or keep a crate off the ground where the other rooster can't get face time with the one locked up.
