[everforward]

It's very hard to create a force that will incapacitate someone without a substantial risk of killing them.

Sufficient blunt force to some area to stun someone will kill them if it hits elsewhere.

Tolerances for electricity are different between people (Tasers do kill people).

Mace and pepperballs don't fully incapacitate someone.

Lightning guns (or really any directed energy weapon) have problems with exponential decay.

Tranquilizer guns might work, although I don't know how wide the dosing range is.

It's a hard problem. People still die from anesthesia, which is basically trying to keep someone incapacitated in a very controlled setting.