Yes, there are policies with some cruise lines that stipulate a U.S. agent cannot sell a cruise to someone outside of the USA, Canada, or Mexico. However, as with any policy there are always exceptions.
With some cruise lines, if an agent goes against the policy, there will be no problems with the reservation, but the agent will not receive their commission.
With other cruise lines, the policy is more of a recommendation that something engraved in concrete. And with others, it will depend on the individual circumstances. For example, I have a long-time client who lives in South Africa and books 2 cruises a year with me on Celebrity. Celebrity's policy normally doesn't permit this, but because he's not a new client, they have no problem with him booking through me.
On the other hand, Princess outright forbids it. I cannot book an international client on Princess, period.
So this is not a simple yes you can or no you can't. It depends on the cruise line and often the specific situation as to whether it can be done or not.