How do butterflies defend themselves from other creatures when they don't have the ability to sting or some other offensive/defensive mechanism?
Butterflies have a number of defence mechanisms, even if they don't have the ability to sting (as adults - see spines in larvae below). Here is a non-comprehensive list of some defensive mechanisms:


Toxins or repellents, which can be present in both adults and larvae.
Aposematism (warning coloration)
Mimicry, both in the form of resembling other poisonous species and using features such as eyespots.
Spines, bristles or long hairs, often in larvae (see google images), to make ingestion highly unpleasent or dangerous. See the Pine processionary moth (Thaumetopoea pityocampa) for an nice example of defensive larval hairs.
Camouflage/hiding
Sound clicks used to startle or disorient predators. This is most often known from moths to prevent bat attacks (see e.g. Cycnia tenera and Fullard et al, 1994).
Conspicuous features at the edge of wings to make them targets of attack, e.g. eyespots or appendages, which are used to lure predators to attack non-essential body parts (see Olofsson et al, 2013 for an example).