There are patents. There is taboo locations
There is also laws that will be created that limit the legal use of certain weapons. These each use implicit and explicit contracts to regulate.

But don't expect an equivalent of a nuke around 300 AD...

Don't even expect equivalent of TNT or A Pipe bomb for even a 5 or 6 real life years. 250-300 in game years passed.

This all depends on how realistic they want to be with tech of course, or if tech suddenly uses magic instead of electricity or mechanical energy.

Assuming you were the king of your people, here is how you could set that up:

You would have an implicit contract as law that prohibits the use of said military ordnance on a kingdom wide level - signed by you as the king.
Everyone in your kingdom will have to abide by this law.

Explicit contracts can be signed by the combination of two legal bodies as follows:

• Individual - Individual
• Individual - Group
• Group - Group

Then you'll need an explicit contract signed by you as the king and a representative of the craftsmen in charge of the construction that grants an exclusive right to be in possession of military ordnance.

And a third explicit contract signed by you as the king and the representative of the military about having exclusive rights to have and use said military ordnance.

For all mentioned contracts you'll need sufficiently skilled scribes who can formulate/craft those contracts.

@Iota
Here's the most recent information on the patent technolgy mechanic being cut from scope and how it works as of now.

So it still exists in a certain form for guilds, but it is not the same as described way back in the Design Journal 13.

For more details on how to prevent certain people from using various technologies, we'll have to wait for the new system to be presented to us.

However, as mentioned by Gunghoe, you'll be able to restrict access to buildings by leasing it to an organization (for example a guild or school) and also create extra keys for them to access the locked place:

"In addition to residences and inns it's also an obvious system for creating guild halls. You can use the access system to control who can legally enter which rooms, giving you the opportunity to have private libraries, armories, or other things which some guild members shouldn't have access to." - from DJ 15

That way it is more difficult for people to get a hold of your devices or weapons legally.