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.