Two Just Stop Oil protesters must pay Madame Tussauds £3,500 in compensation after slapping vegan chocolate cakes in the face of a waxwork of the King.
Westminster Magistrates' Court heard they targeted the model "in the manner of a slapstick comedy" last October.
The activists smeared the cakes, which were topped with shaving foam, in the model's face during the incident.
Eilidh McFadden, 20, and Tom Johnson, 29, denied criminal damage but were convicted on Tuesday.
His waxwork needed to be cleaned and repainted, while its jacket, shirt and bow tie had to be dry-cleaned.
The royal set at the tourist attraction, including a red throne, also had to be cleaned.
The model, along with those of the Queen Consort and the Prince and Princess of Wales - which can cost between £75,000 and £200,000 to make - were removed from display until the following morning.
The court heard some staff had to work an extra five hours, while admission was halted for almost an hour, which could have prevented up to 900 people entering during the busy half-term holiday.
Johnson told the court: "The substance was carefully chosen to inflict as little damage as possible", while McFadden added: "Shaving foam on a waxwork is nothing compared to the damage we see from the climate crisis."
'Totally trivial action'
However, prosecutor Jonathan Bryan said: "Your actions have done nothing whatsoever to help those persons affected by climate change, have they?"
He added: "Putting a custard pie on a waxwork model of King Charles is not going to convince anyone about climate change, it is a totally trivial action."
District Judge Neeta Minhas convicted them of causing £3,500 of criminal damage, telling the pair she was satisfied the damage was "significant" and "not minor or temporary".
Self-employed artist Johnson was given a 12-month conditional discharge and ordered to pay £1,750 compensation and £250 costs.
McFadden, who has three previous convictions for aggravated trespass, was ordered to pay the same amount of compensation and costs, and handed a 12-month community order, including 80 hours of unpaid work.
[posted to Non-WAM as pieing an inanimate object isn't really WAM to me]
[posted to Non-WAM as pieing an inanimate object isn't really WAM to me]
Good judgment on your part. Here and there, I've pushed my own Barbie dolls into a mud pit...it's true that you can get interesting pictures off that, but, nah, I'm with you. It doesn't quite cut the fat.
Love you, too
