For context- there is a family friend who has autism. This man used to live with his father, who was his legal guardian. Upon his father's death, a family member took over taking care of him. This person stopped taking care of him, and instead is letting two strangers live in the home, provided they take care of him. We are worried that these strangers are possibly abusing him, and have filed a complaint with the proper authorities. His legal guardian has said that they will no longer allow us to speak with him. My question is: Does this person have the right to stop him from speaking with us? This is in Kansas, in the USA. The autistic man is an adult.
This page gives a broad overview of guardianship and visitation law in the US. Some states have stronger restrictions on a guardian's control over visitation, so in Tennessee only the court can remove an individual's right to communication, visitation, or interaction. There is a "uniform law", Uniform Guardianship, Conservatorship and Other Protective Arrangements Act which would limit a guardian's power to limit visits, but this has not been enacted by Kansas, and the Kansas does not have any such laws (art 30), so it is a power of the conservator to restrict visitors.
In general, a guardian does have the power to prevent their ward from speaking to particular people. In general, a guardian has full authority over the person of the ward in much the same way that a parent does with respect to their minor child, so long as they do not engage in abuse or neglect.
But, if there is suspicion of abuse, there may be means to cause a third-party to inquire regarding the allegations. In the case of a child, the inquiry would be usually made by child protective services.
I am not sure which agency would have authority to investigate allegations of potential harm most effectively in Kansas, but it sounds from the question as if you did locate that agency and make a complaint.