Praise be to Allah.
There is nothing wrong with your saying “You have knocked so-and-so off the throne” to refer to someone defeating someone else in a game. Thrones may be attributed to created beings in either a literal or figurative sense. Kings have thrones, and it may be said that a king ascended the throne, or gave up his throne. Allah, may He be exalted, says concerning Yoosuf (peace be upon him) (interpretation of the meaning):
“And he raised his parents to the throne and they fell down before him prostrate. And he said: ‘O my father! This is the interpretation of my dream aforetime! My Lord has made it come true’”
And He says concerning Balqees (interpretation of the meaning):
“I found a woman ruling over them", she has been given all things that could be possessed by any ruler of the earth, and she has a great throne”
And it may be said (in Arabic) that a person “ascended the throne of the game” which is a metaphorical expression, meaning that he is the winner, and if he is defeated, it is said that So-and-so knocked him off his throne or caused him to fall from the throne. There is nothing wrong with that, and it has no connection to the Throne of the Most Merciful, may He be blessed and exalted, which is the highest and most sublime of created things, and is the roof of the created universe.
Allah, may He be exalted, has described His Throne as being mighty or supreme, as He says (interpretation of the meaning):
“Allâh, Lâ ilâha illa Huwa (none has the right to be worshipped but He), the Lord of the Supreme Throne”
We advise you to beware of waswasah (whispers from the Shaytaan) regarding this and other matters, for it is a grave disease and a great evil. The best remedy for that is to pay no attention to it, whilst remembering Allah, may He be exalted, a great deal and turning to Him.
And Allah knows best.