Fraid' not. You're bound by the terms and conditions to see it through til' the end unfortunately. It might be worth seeing if you can downgrade your tariff and speaking with the O2 Home Broadband Sales team to find out if you could maybe get some sort of discount for having O2 Mobile Broadband too. Worth a try eh?
I do remember seeing a discounted price being available for customers who have both mobile and home broadband. You might want to ask if this is still an option?