You should find that the thread you have stripped is actually a helicoil as it is good engineering practice to fit helicoils in aluminium where the fastener (or in this case, the spark plug) is to be frequently removed and replaced. You will usually find them in cast iron heads as well, for the same reasons. It should be a matter of removing the old helicoil and fitting a new one. To do the job properly you really need special equipment.
The helicoil is effectively a diamond section wire wound to give a spark plug thread on the inside and a bigger (and therefore stronger) thread on the outside where it screws into the cylinder head. Problems can (and do) occur if its the helicoil to head interface that has stripped, rather than the helicoil to plug threads. This is why it is important to keep your spark plug wells clean and clear of debris before removing plugs and to ensure the plug threads are clean before re-fitting them. It is vital that you do not over-tighten the spark plugs - stick to the recommended torque figures.
Incidentally, Helicoil is a registered trademark of Armstrong Fastenings Ltd who are pretty much the UK defacto industry standard. Other screw thread inserts are also available
Best regards
Stuart