Cancelling the online banking facility seems a bit of a tough route to take. You won't be the first (or the last) to forget your login information so just call your bank, go into branch or use live chat on their website, explain the problem and ask them to send you new login details.
Online banking is too useful to give up. I use it all the time to keep track on spending and also to check that nothing is leaving my account that I didn't authorise.
I can also make payments to other peoples accounts (I pay my daughter's pocket money into her account using this method) and can easily set up or cancel direct debits.
Banks are very helpful, especially as they have so much competition and always want to be the best, so customer service is very important to them. Don't be worried about talking to them, they are there to help.
If for any reason you do decide to cancel the online banking facility I am sure you could tell them and they can do that for you but think about it first, it is a very useful service.