Hi there.
We have this problem all the time with our customers. The policy holders take it upon themselves to cancel their own direct debit, which by it's nature, will finish after the last installment is collected anyway. People regularly fail to read the documents and know the difference between a direct debit and a standing order.
If you have cancelled your payment the insurer may well have cancelled your policy and will want the last installment. If they haven't cancelled it and your policy ran for thr full term then it'll be the last payment they're after, They cannot charge you a cancellation fee if your policy runs for the whole year and you then choose not to renew.