How are they taking the fees? Via card payment or direct debit.
If it’s a continuous payment Monzo should be able to remove the authorisation. It is still your responsibility to ensure you’re not in a contract with the company though.
If worst comes to worst, get a new card and new card number.
You’ll need to still speak to the company as Monzo won’t sort out any disagreement you have with them.
This would’ve been a lot easier if you’d just said the full detail from the start.
It sounds like you’ve signed up to a service and there’s a Continuous Payment Authority in play. Monzo can absolutely help here and block it, or you can freeze the card/update it and it should stop it.
I have had this in a past with a company which despite requests would not stop payments. You do need to let them know as they could still insist you owe the money.
My final advice would be to read T&Cs carefully in future even for trials.