I cancelled by direct debit. Can they still take money?

Basically I took out a contract on an iPhone 4 with O2 last year.
Money became an issue so I just cancelled my direct debit with Lloyds TSB
Will they still be able to withdraw money from my bank monthly regardless?

Probably not but they can come after you for breech of contract.

If it's a direct debit to a bank account, no, they can't - but they can use debt collectors. If it's a recurring transaction on a credit card, yes, they can continue to take payments but your bank will be able to reverse them

Often times banks will continue with certain payments it varies from bank to bank. Generally no however I can guarantee you will be sued, you will loose and you will pay

Yes they will keep putting in the direct debit to be paid and it will get returned and you will be charged by the bank unless you actually cancel it with o2 aswell. It no good just thinking by cancelling it in the bank that its the end of story

Cancelling the direct debit with your bank won't work. As you've signed an agreement to pay the direct debit, your liable for any payments due. O2 will just request the payment again and it'll come out.
You'll need to contact 02 to discuss this with them. As you're in a contract, you'll still owe them the money. They may even ask for the iPhone back if this was part of the deal.
If you miss your payments to them this could impact your credit record which will affect your ability to be accepted for loans, credit cards and mortgages in future.
They will also be entitled to apply to get a County Court Judgement to claim the funds from you and send the bailiffs around depending on how severely they react.

Bottom line, contact 02 and see what they can negotiate.