Is there anyway of stopping my son using his data once he has hit his limit as i note he has been charged £2 already for going over?
unfortunately Virgin Mobile don't "cap" data allowances
Once the allowance is used up if your son continues to use mobile data you'll be charged at the daily rate of £2 per day.(As you've found )
You may be able to set a mobile data limit on the device itself.
You can certainly make him aware of the limit and keep a check on it using the "My Account" app.
Once reached, he can turn off the data from the Settings on the handset to avoid further charges until the next refresh date.
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