You can decide on the instructions to the bank when you set up a joint account. Typically in a company it will be any two of a list of signatures, in a club it would be at least two etc etc for good governance. However, in a personal case, it is likely to have each named account holder have their own chequebook, and it is very common to allow either signature fill in a cheque. The alternative would be to have your partner sign cheques in advance so that you could write a cheque when you need to, which clearly isn't exactly secure.
I suppose, what's strange here is that while I can clean out the account, I can't stop the bank charge unless we both sign the form !