When a mission to the ISS ends and the astronauts/cosmonauts shall leave the ISS with the Soyuz/Dragon spacecraft, what if an astronaut refuses to enter the Soyuz? What if he/she wants to remain on the ISS? Does he have a right to do so, or would his mates have to force him with violence to enter the docked spacecraft? Is there a procedure for the case an astronaut is unwilling to return to Earth?
I doubt there's an explicit procedure to follow in this case, but such a crew member would doubtless be physically forced to return.
Any crew member who would refuse to return would be untrustworthy and an obvious danger to the other crew members. Leaving that person on the ISS would be far too dangerous.
The seats on spacecraft going up and down to and from the station are allocated years in advance. Assuming the rogue crew member hoped to return to Earth eventually, they'd have to bump a future astronaut from coming up in order to have a seat free to return; I doubt any other crew member would acknowledge a right for them to do that given how hard every one of them worked for their seat.
Assuming that the other crew members were able to physically overpower the rogue crew member, duct tape would be employed to "mummify" them and secure them to their seat in the Soyuz to avoid any risk of them interfering with the return flight.
The only scenario I can imagine in which case the other crew members might conceivably allow someone to stay aboard ISS is if the astronaut was claiming political asylum because they feared they would be killed by their home country after their return to Earth.
In your hypothetical, if a Russian cosmonaut didn't want to return to Russia, but another return method was available (e.g. a Crew Dragon landing under US authority instead of Soyuz landing under Russian authority) I could conceive of the ISS crew agreeing to an exchange of crew slots in order to save a life, but it's certain that all of the crew members who went along with the scheme would face severe career consequences -- none would ever fly to space again (at least 4 US astronauts lost their flight privileges for much more minor offenses (Carpenter on Mercury-Atlas 7; Schirra, Eisele, Cunningham on Apollo 7)). Given the current US administration's spineless deference to Russia, the cosmonaut would be well advised to try and stay aboard at least until January 2021.