Not sure about petrol injectors but most diesel injectors are held in by a claw and torqued bolt. The claw simply holds the injector in place, the o-ring makes sure there's no leakage.
Diesel injectors can be awkward to get out, you can use penetrating fluid, run the engine with a blanket over the offending injector and let the compression blow it out or where you have a screw thread on the top of the injector fashion a slide hammer with a nut welded to a bar with a 2/3lb lump sliding up to a stop.
As I said not sure about petrol injectors, if yours is a non-screw injector the above may be applicable - if it's a screw thread then penetrating fluid and a ring socket, a flat socket will just round it.