The only way that a direct debit can happen is if you have automatic membership renewal enabled.
There is no other way for 48 to automatically draw payments from your account other than this or if you explicitly make a payment/purchase/topup online yourself.
If you have automatic membership renewal enabled and want to stop this monthly payment then simply disable auto renewal.
http://community.48months.ie/t5/Most-Popular-Questions/Automatic-membership-renewal/td-p/32390
Hope this helps.