By not allowing them to and reinforcing calm and desired behavior. Re-direct them to the desired behavior and reward. Always reward a calm dog.
Most undesired behavior is because of the human (exception may occur but are the minority of cases). In your mixed pack, you are the pack leader, and the pack leader sets the tone and behavior standard. If there is undesired behavior, it most probably is because of insufficient pack leadership (direction and protection).
Lots of reason can contribute to your dog being aggressive (case dependent). But simply put, most of the behavioral cases the dog feels uncomfortable/uncertain in situations or is not well socialized in handling certain situations. You as the human also determine the dog’s reaction in most of those situations. A calm socialized human will lead to a calm socialized dog (Pack Leadership – Direct and Protect).
Dogs are animals first, then species (breed and name is human concepts). Under species they are pack animals stemming from the common grey wolf. This means they have an instinctual need for being in a pack. Mixed packs (human and dog’s) with only 2 – 4 members tends to lose out on tapping into pack dynamics. As soon as we introduce MOST dogs to our pack, they quickly pick up on the pack language (verbal and non-verbal), and then easily find they slot in the pack. When facilitating this with walks, play and feeding; the pack mostly does all the work to socialize and help rehabilitate the temporary member.