to depose, to dethrone

Vgl.

Court deals decisive blow to deposed Greek royals

Court deals decisive blow to deposed Greek royals | World news | The Guardian

Dictionary: unseat wäre wohl noch eine Möglichkeit ...

Battle of Evesham

https://en.citizendium.org/wiki/Battle_of_Evesham

20 Feb 2009 — For a period of time, the barons of England, led by Simon de Montfort, in a civil war unseated the King of England in the late 13th century, ...

napoleon's reforms

Commack School District

https://www.commackschools.org/Downloads/napo...

DOC

For example, he unseated the King of Spain, and placed his own brother Joseph Bonaparte on the throne. Napoleon divorced his first wife, and married a ...

https://www.theguardian.com/world/2008/jun/11...

The deposed Nepalese king, Gyanendra, today waved farewell to the salmon-pink Narayanhity royal palace in Kathamandu, vowing never to flee the country and denying persistent rumours that he had a hand in the 2001 royal massacre that saw him claim the throne.

https://www.theguardian.com/world/2013/dec/15...

The deposed monarch, who was forced to flee Athens shortly after the seizure of power by a group of army officers in 1967, has stunned Greeks – and most of his relatives in the royal households of Europe – by resettling in the capital where he was born and schooled.

https://www.theguardian.com/world/2012/jul/13...

Thai court dismisses claims ruling party was trying to depose king

https://www.theguardian.com/world/2023/jan/11...

Of the seven Greek monarchs of the 19th and 20th centuries, three were deposed, one assassinated, two abdicated and one died of septicaemia after being bitten by a barbary ape in the royal gardens.

Depose is the usual word.

The Guardian has 4,400 results for 'depose' + 'king' and only 515 for 'dethrone' + 'king'. The latter results are often less literal, eg https://www.theguardian.com/music/musicblog/2... "Should the BPI dethrone Jonathan King?", or https://www.theguardian.com/world/2015/mar/02... "Brazil's king of deforestation dethroned in drive to beat land clearers")

Schönes Beispiel!

Jonathan King und alle anderen mit diesem Nachnamen werden dethroned...

Mitte 18. bis Mitte 19. Jh. war wohl eine schlechte Zeit für Könige :

https://books.google.com/ngrams/graph?content...

deposed=>king,unseated=>king,dethroned=>king,depose=>king,unseat=>king,dethrone=>king

Of the various suggestions so far the only one I would use in the OP’s context is “depose”:

“The king was declared mad and deposed.”

What hasn’t been suggested so far, and which, I find, could work well here is “remove from the throne”:

“The king was declared mad and removed from the throne.”

(Which I ultimately settled on would depend on the surrounding language.)

I agree with Bion and papousek - in the OP's context, "deposed" fits best

"removed from the throne" would also be possible but "unseated" doesn't work for me. Sounds more like Henry VIII coming off his horse when jousting in 1536.