Firstly you would need to deactivate HulloMail to stop HulloMail from handling your voicemail:

For iPhone please dial the 'HulloMail Deactivate' contact from your iPhone contacts.

For Android please select from within the HulloMail app:

Menu -> Settings -> Voicemail Activation -> Deactivate Voicemail

Please bear in mind that the deactivation number is carrier specific.

Once deactivated you can close you account.

For iPhone please select; Menu -> About -> Service Termination -> Close Account

For Android please select; Menu -> Settings -> About -> Service Termination -> Terminate Account

Once the service has been terminated, please remove the app from your handset.