Isn't there an analogy to be had in the relationship between the Federation and the Romulans?
Sure, we could place a Romulan neutral zone-like barrier on the American southern border, while the occasional ship did get through, the Romulans did not simply flow across.
Greece could have one on their northern border too, where they're currently building a giant physical barrier. A similar barrier could stop all those fukking rockets that the Palestinians keep firing into Israel. Another between Afghanistan and Pakistan (actual India might like one too). Don't forget Taiwan.
Good fences make for good neighbors.
Well, we could start by having all nations embrace both the Universal Declaration of Human Rights
Are you kidding, have you ever actual read that thing? It's the death of national sovereignty, in over sixty years less than a third of all the nations have signed it. Please.
as well as Trekkish secularism.
From the Universal Declaration of Human Rights
Everyone has the right to freedom of thought, conscience and religion
[snip] freedom, either alone or in community with others and in public or private, to manifest his religion or belief in teaching, practice, worship and observance.
Maybe instead you'd like all nations to embrace just bit and pieces of the declaration?