1) If someone has weapons in outer space, the only way to stop them would be to bring weapons into space and confront them. Otherwise they are free to do what they will with their weapons.
2) Until humanity matures past our possession based society there will always be conflict and thus a need for protection, or weapons. We have more than enough experience here on Earth to show that diplomacy is not sufficient.
3) The definition of weapon is different in everyone's interpretation. For example, In my opinion spy and even GPS satellites are a weapon. They are part of an arsenal used by a military regime to gain a battlefield advantage. An active military individual could also be considered a weapon. Should we allow a military colonel to fly a space mission?
4) Are our 'weapons' definitions going to echo the strict restrictions of air travel in some countries, better leave that nail clipper home for that 3 month trip to space.
Like it or not, weapons are already in space and in use. as stated earlier, until we change global society these weapons and others will be a reality in space. We have all kinds of "laws" at all kinds of levels and we cant police it here on Earth. What naive thinking leads us to the delusion that we can prevent weapons from going to space?