There's been a lot of discussion over the years about how these things come about. The most obvious is that you may have taken up a free trial offer and then not cancelled it at the end of the free period. However, it also seems that interacting with an advert or pop-up can trigger it - the originator gets your number from the phone and sets up a charge. In the case of premium-rate texts, it may also be that random numbers are entered into a remote system.
Because much of these services originate outside the UK, the regulator is sometimes limited in what they can do. Networks can also have a problem cancelling because they receive an apparently legitimate incoming charge which they simply pass on. However, where there is bad practice, they will sometimes block a sender completely.
Your best bet is to set up the charge to bill bar on your Vodafone account. Some people like to use theh facility, but many don't need it and it's a good first line of defence. I'm not sure whether it works retrospectively, but it's worth a try.
You can also block premium rate texts, but this bar affects outgoing as well as incoming and stops you texting all shortcodes, which can be a major inconvenience. (The way round that is to set the bar and then remove it temporarily when necessary, but a more granular bar would be desirable.)
