Opponents of open borders and of immigration at large draw from a variety of philosophies, moral theories, and normative ethical traditions. Note that even if you adopt these philosophical stances, the anti-immigration conclusions don’t follow automatically but still need to be argued.
Killing versus letting die
Supporters of the right to migrate argue that preventing people from migrating is an act of coercion. A common retort to this view is to draw on the distinction between killing and letting die, or more generally, the act-omission distinction. This notes, first, that there is no general obligation to rescue people who are dying as long as one is not directly responsible for causing their death. It is then argued that forbidding people from migrating can be better thought of as choosing not to help rather than actively hurting or coercing the prospective migrants.
Citizenism is the name coined by California-based writer Steve Sailer for the view that individuals have considerably greater obligations to their fellow citizens than to non-citizens. In the context of migration restrictions, citizenism would argue that migration policy should be set for the benefit of, and taking into account the views of, current citizens rather than prospective migrants. Citizenist defenders of immigration restrictions argue that nations have no moral obligation to allow migration, although there may be particular situations where it is beneficial for their citizenry to allow various forms of migration.
Citizenism can be thought of as a particular form of communitarianism that gives particular weight to community as defined by formal citizenship.
Collective property rights
Closely related to citizenism, collective property rights refers to the idea that nation-states have collective property rights over the common land and public resources, and it is hence legitimate for them to forbid people arbitrarily from access to these common lands. The collective property rights argument is not about the wisdom of immigration restrictions, but rather, is a claim that no justification needs to be provided for immigration decisions at all, just as a private property owner need provide no justification for his decision on whom he invites to his home.
A more niche variant of collective property rights, pioneered by anarcho-capitalist philosophers Murray Rothbard and Hans-Herrmann Hoppe, is the anarcho-capitalist counterfactual.
Territorialism is a slight variant of citizenism. It refers to the view that nation-states are obliged to defend the interests of people currently in the geographical territory of the nation-state, but have considerably fewer moral obligations to those outside. Territorialism differs from citizenism primarily in the way it treats non-citizen residents, guest workers, and illegal immigrants.
Local inequality aversion
The term local inequality aversion refers to the view that local inequality is bad per se. Insofar as free migration increases local inequality in sending or receiving countries, it is considered bad, even if overall poverty and global inequality are reduced.