The concern about habituating predators to being fed by divers and the potential for a modification of their normal behavior is a significant issue. However, there is pretty much unanimous opinion that their removal from foreign ecosystems is to be encouraged. Florida has a number of incentives and promotional programs directed toward their removal.
Lionfish have excellent meat, so many people are interested in keeping them for their own use. If lionfish are speared and placed in a container and not fed to the sharks, normally any sharks present will not become too interested.
If they do become too aggressive, then one potential solution is to send the lionfish container to the surface with a marker buoy. This will only work where you have a boat crew who is able and willing to retrieve the container and the floating marker. Separating yourself from the source of any blood and visual stimulus usually causes the predators to hunt elsewhere pretty quickly.
After the dive, if you don't want to eat the lionfish, they can be disposed of somewhere away from the dive site so that predators are not able to associate food with divers.
If you really don't want to keep the lionfish for food and you don't want to carry a container, then it is often possible to spear the fish and then quickly slide the fish off and place it under a ledge without being too conspicuous. For example, barbless, multi-prong tips are available that are quite effective on lionfish, but will also allow the dead fish to be quickly stripped off the spear tip with minimal effort. This may help to reduce the opportunity for predators to make the connection between diver and food.
Obviously if a predator is already following you and looking over your shoulder, it might be time to give the fish a pass.
Spearfishing has always increased the potential for predators becoming interested in the catch, but compared to many other types of fish, lionfish aren't generally going to be a problem in most areas because they normally are not struggling and releasing a considerable amount of blood.
Also with respect to concern for impacts to the reef, if the diver uses a small pole spear or other device intended for lionfish removal, they are designed to have limited power - so they should be very easy to control. In addition, as with any kind of hunting/shooting activity, the hunter must carefully identify the target and be aware of what is behind the target. With lionfish, it doesn't take a whole lot of effort to carefully line up your shot, so that a shot that penetrates (or even misses) will not harm the reef.
A diver who has good control of their buoyancy and position in the water, should be able to learn to safely harvest lionfish pretty quickly since the necessary gear is not cumbersome.