I made the same enquiry 4th Feb to the forum..... In my case I was being greedy and also after a reasonable number to use...... however Voipfone Support came back to me very quickly and told me to email them with the numbers I wanted removing, so it isn't a problem as such.... I know what you mean though, if you can cancel/remove geographic numbers on-line why not the same with the others.
I know what you mean as well, i may require a number for a month and then don't want it on my account any more. I don't want to have to e-mail VOIPFONE support to get it removed i feel this option needs to be added to the account page.