@djmixter What phone do you have? It may have an in built option to reject unknown numbers? IF not, then as somebody else suggests, download an app that will block all unknown numbers.
If they do not get bored and keep on ringing, then you can always request a new number from giffgaff here https://giffgaff.com/profile/details#msisdn but if you do this, you cannot retrieve the old number.
Also be aware, if you use an iPhone, people can contact/message/phone you via the Apple iD username. So if you do have an iPhone, just be aware of this - you may need to change this Apple iD if things persist.