cocoflan
You could run the DietPi-PREP script:
bash -c "$(curl -sSL https://raw.githubusercontent.com/MichaIng/DietPi/master/PREP_SYSTEM_FOR_DIETPI.sh)"
At least regarding DietPi scripts/configs and APT packages it resets the system somehow. But many manually installed files will stay in place, e.g. things in /opt/ or /usr/local/, all that was installed as source build or generally outside of APT packages.
So at least it would be recommended to uninstall all software via dietpi-software first.
Taking into account that there will always be remains, I agree with ghettopi that one should simply re-flash the SDcard as a much cleaner approach, if you really want a 100% reset.