These are NOT reverse premium texts. They are Payforit subscription charges. Blocking the texts will not block the charges. The charges are applied directly to your account (and taken from the airtime credit). The texts are usually prefixed "Freemsg:" to indicate that they are not charged. The texts tell you who is taking your money and give instructions on how to stop the charges. Blocking them is futile and deprives you of the information you need to stop the charges.
As these are subscriptions, it is important that you send the STOP text (or phone the number given in the text). If you don't do this further charges will be taken each week.