I'm thinking of putting a clause in my money back guarantee saying that if X amount of the account's bandwidth or more has been used a refund can not be offered.

First, what kind of amount should I put? 1/2?

This is really only to avoid any issues in the future where someone might abuse it by using all the bandwidth within the 10 day period and then asking for a full refund.

Thanks :)

I would use this:
If you use more than 60% of your allocated data transfer limit you will not be eligible for a refund.

I think when you first start a new company and until you become established, you need some type of guarantee. Some people will try and abuse it so as soon as you become established like lets take for example servint.com, they do not offer a guarantee and they even post why on their website but again, they can get away with this because their reputation proceeds them. I would make it quite clear like zeonfx.com stated and post some sort of limit.