While it did include roads, the Ho Chi Minh trail (note the word trail) also utilized smaller pathways for foot traffic and biking traffic, also it should be noted that much of the trail runs through a different country. Vietnam was a proxy war meaning the civil war in Vietnam to make it communist or a democracy was being influenced by foreign powers (China and Russia for north Vietnam, and the United States for the south). The United States, if it were to set up road blocks would violate the neutrality of Laos and Cambodia. This could case a larger war with nations not being impacted by the same ideological struggle who could then possibly side with the north Vietnamese. Another reason possibly is because this would cause the United States to lose face and look like war mongers who are trying to invade Southeast Asia. Lastly setting up defensive checkpoints plays right into how the north utilized guerilla warfare. Having men set up checkpoint on small trails surrounded by jungle and rough hilly terrain sounds ripe for an RPG-ing to me and the treat of ambush would be too great.
The Ho Chi Minh Trail was not a literal road. It was an informally known network of roads, trails and bike paths that ran through dense jungle on the western side of Vietnam, as well as through neighboring countries of Cambodia and Laos. People’s Army soldiers would travel south to recruit and supply insurgent groups known popularly as the Viet Cong. Publicly, there were military units dedicated to tracking down North Vietnamese troops in south Vietnam as they entered the country. Privately, some of those troops carried out actions in Cambodia and Laos. Legally speaking, as the United States had not declared war with Cambodia or Laos, all of these actions were illegal. Throughout the 60’s and early 70’s, the Johnson and Nixon administrations ordered airstrikes on targets in Laos and Cambodia, but always with a veil of plausible deniability. Even today, land mines and unexplored ordinance left over from these illegal actions are a huge risk to the population of both countries. So, to answer your question more directly, there were too many paths to try to block with road blocks, but they certainly did try to interrupt the flow of munitions and troops to the south using some conventional methods.