Samak had hosted a popular TV cooking show called Tasting and Complaining before becoming prime minister seven months ago.
Samak made a few appearances on the show - a mix of cooking and rants on various topics - after taking office, prompting a group of senators to petition the court on grounds that a prime minister was not allowed under the constitution to work with private companies.
The case is the latest embarrassment for Samak, who has not been able to enter his office at Government House in the capital, Bangkok, since protesters stormed the compound on August 26.
They say they will not leave until he resigns.
There appeared to be some hope of ending the standoff on Monday as a new mediation effort by parliament was due to get under way.
Senate speaker Prasopsuk Boondet, who was appointed mediator by parliament last week, was to meet with Thailand's opposition leader and the heads of the six political parties that make up Samak's ruling coalition government.
|Samak's show was a a mix of cooking and rants on various topics [Reuters]
"I am optimistic that there is still a way out of the ongoing stalemate," Prasopsuk told reporters before the meeting.
But the meeting does not include the People's Alliance for Democracy (PAD) which is behind the protests and says it will not negotiate unless Samak resigns.
The PAD accuses Samak and his government of corruption and violating the constitution.
The protesters also accuse Samak of being a stooge of Thaksin Shinawatra, who was prime minister for six years before being deposed in 2006 following months of street protests by the alliance.
On Sunday, Samak denied a rift with the army and dismissed the possibility of another coup.
"The military will not stage a coup," he said in his weekly Sunday television broadcast. "They know the international community will not tolerate [another] coup."
Confident to travel
The premier, who has steadfastly refused to quit, said he still planned to travel to New York this month to attend the United Nations General Assembly meetings.
Samak noted that Thaksin was ousted in 2006 while he was in New York at the UN headquarters, but he was confident that this would not happen to him.
"I will deliver my speech at the United Nations and I don't believe any unwanted incident will happen like when Thaksin went to speak there," he said.
Samak imposed a state of emergency last Tuesday after violent clashes between his supporters and opponents left one person dead.
Calm was quickly restored, but Samak has acknowledged that the emergency decree has been ineffective at removing protesters from Government House.
"After declaring the state of emergency, I thought everyone would be scared and disperse. But no one cares. They're ignoring the law," he said.
"But I will not bow to them."