For one, you could detect these no-knocking individuals quite easily:
- Cameras on the exterior of the property
- Motion lights, motion sensors, motion bells are all quite cheap.
And re: destruction/obfuscation of electronic evidence-
-"One-click" destruction scripts, etc.
- TrueCrypt volumes, everywhere. Circuit breakers, power strips, etc. with switches. If you don't manage to destroy the volume, no aggressor will be able to access it.
- Degaussing setups for spinning disks, activated by similar mechanisms.
This stuff is pretty cheap, easily available. Anybody caught by these technically-incompetent police executing no-knock raids deserves to be caught.
Hell, if you don't have a few 6TB volumes full of random bits, labeled "Evidence," you aren't doing it right. Gotta tie up those investigative resources somehow.
Which would be exactly what's not happening if "a guy has a gun".
Either someone is in danger, in which case no-knock isn't needed, or they're not, in which case why the hell is a SWAT raid being carried out on the basis of an anonymous tip?
The classic example is if the police think that evidence could be destroyed very quickly, e.g., electronic records.