Ok. Assuming that your steam head removes very easily since you stated you've had to replace the diffuser many times, then there is an alternate to the steam guard I first suggested. It is important that your head removes easily as we do not want to create a leak by loosening a pipe in the wall.
You can look to replacing your old steam head with the new Steamist 3199 steam head that is self diffusing. This new steam head is 3/4 inch NPT and if you have a 1/2 inch steam line, then you will need a bushing for the size transition.
The cost for a new steam head will be close to the steam guard I suggested, so you may wish to err on the side of the easiest solution and purchase the steam guard.
Here is the steam head info Steam Head 3199
Check out Bushing