The hammer equivalent of "caulk" would be the skybox texture. But any solid texture can work as caulk would in the quake engines. You just need to prevent leaks by not having gaps or transparent brushes surrounding your world.
If by caulk you mean generic level sealing texture, use nodraw. The only difference between nodraw and skybox textures is that the skybox textures generate light if you have a light_env entity in your map.
Remember that you need to place nodraw brushed behind displacement maps in order to seal of a level properly, as displacements do not block vis or seal levels from the void.