The one that is an example of a social barrier to cultural diffusion is : Laws that prohibit western-style clothing This will prevent a lot of western people to enter that specific region because they're afraid to get into trouble
Cultural diffusion is the spread of cultural beliefs and social activities from one group of people to another. The goal of cultural diffusion is to broaden people's minds and horizons and make them more culturally rich.
Laws that prohibit western-style clothing are a social barrier to cultural diffusion because they limit a person's availability to get in touch with new cultures, actions or ideas. Clothing is a matter of choice and some people may not want to adjust their style of clothing under any given circumstances. It does not mean disregard or lack of respect for other cultures and their particular attires, it is simply a social barrier that not everybody would be prepared to cross.
They are expressed as executive orders. The president can make these for other things too like limiting immigration or making treaties with foreign nations. They can be deemed unconstitutional and abolished by the supreme court, but they don't need the approval of the congress.