Can I hide tax / VAT on our invoices?

Nope, most countries require you to state tax on invoices even if it's just to say that none applies to avoid any ambiguity.

Therefore, Moobiz will always display and total the tax on an invoice.

If you don't need to apply tax to your customers, you may also want (or be required) to include a message stating this such as 'MyCompanyName is VAT exempt and therefore VAT has not been applied to this invoice.' for example.

This is fairly easy to do, just click Account Settings at the bottom of any page and then Manage Templates. There you'll find an option for including text in the footer of all invoices where you can add a suitable message.