Marshall, Jeamteerasakul and Chachavalpongpun left the country years ago and face arrest if they ever return for their scrutiny of royal practices. As a result of the ban, Marshall and Chachavalpongpun told CNBC that their respective social media following spiked dramatically, but that they remain concerned about the safety of their families back in Thailand.
While the international community has long reprimanded Bangkok for detaining and arresting citizens under lese-majeste and sedition charges, the public targeting of Marshall, Jeamteerasakul and Chachavalpongpun was unusually aggressive, experts said.
"The recent ban is even more heavy-handed than usual and reflects discomfort with criticism of the monarch," said Christian Lewis, Asia associate at political risk consultancy Eurasia Group.
Many believe King Maha Vajiralongkorn, who took over the throne in December following the death of his father King Bhumibol Adulyadej, plays a key role behind Bangkok's increasing autocratic stance. International media have widely described the former crown prince as a playboy who spent much of his time abroad.
Before signing the new constitution on April 6, King Vajiralongkorn made several changes that analysts believe enhance his royal powers, such as naming the monarch as the key arbiter in times of constitutional upheaval. That legally places the king at the forefront of potential political disputes, to which Thailand is no stranger, and sets the stage for the monarchy to intervene in politics, noted Joshua Kurlantzick, senior fellow for Southeast Asia at the Council of Foreign Relations.
Thai officials did not respond to CNBC's request for comment.
"The new reality in Thailand is a move toward enhanced state absolutism, likely reflecting the preferences of the new sovereign, as implemented by the junta," said Paul Chambers, lecturer at Naresuan University in the Thai province of Phitsanulok.
British-born Marshall, a former Reuters deputy editor who resigned from his role in 2011 in order to self-publish an online exposé of the Thai monarchy based on 3,000 leaked U.S. diplomatic cables, alleged to CNBC that it was King Vajiralongkorn who ordered the ban on Jeamteerasakul, Chachavalpongpun and himself.