Stink bugs are a known pest of hibiscus. They don't necessarily smell until they are disturbed or crushed.
Here are some other possibilities:
As far as bio-rational control: hibiscus flowers are sensitive so don't use neem oil, horticultural oil or insecticidal soap products. Pyrethrum (chrysanthemum extract) is effective for contact knock down, but has no residual effectiveness as a protective barrier.
I would choose a a pyrethroid, a synthetic version of the pyrethrum chemistry that has knock down power as well as two or three weeks residual effectiveness. It is low toxicity for mammals, but will kill fish and bees and beneficials if you do indescriminate spraying of the entire garden, but if you don't have massive hibiscus plantings and can spot spray your hibiscus buds and flowers, you will do minimal damage to non-target organisms.