We do not currently have the option to switch off exit tax but DIRT can be avoided by entering your investments and any cash accounts under a Type of Offshore Taxable. Leave the gains rate set to 0%, making the account effectively tax free.
To account for any lump sum inflows that would be usually be deposited into default cash accounts, go to the Savings screen and select your client's default cash account(s) (e.g. John's Cash). Under Advanced Settings > Annually Sweep Balance to Other Accounts, click Add, and add the offshore taxable account as the alternative account into which to sweep lump sum inflows (e.g. selling a home or taking a lump sum distribution from a DC pension).
I should also note that if your client has any income, you could enter it as non-taxable on the Other Income screen.