Sprays rarely work on scale because you need to time it exactly right... they are only vulnerable to sprays during a short time in their lifecycle when they are crawlers, just after hatching. They crawl to their new location and stay put and build a waxy coating to protect them. You'll need to manually remove, which it seems you are already doing. I dip a Q-tip in rubbing alcohol and rub them all off. You'll go through a bunch of Q-tips. You'll have to repeat this procedure every few days until you break the cycle (as others have mentioned).
The citrus leaves are thick enough to withstand the alcohol and the rubbing. You definitely do not want to coppice your citrus. Citrus are good as potted plants because they keep most of their energy above ground and therefore do better than most trees with a restricted root area. Plus, most citrus are grafted on rootstock which you don't want to start sprouting. If you do prune your tree, don't take more that 25% - 30% off.
Scale usually take over when the tree is stressed. They need lots of sun and regular water but wait until the top inch of soil is dry to soak it, and make sure it can drain so it's not sitting in water. They also are high iron and nitrogen feeders because most of their energy is stored in the leaves. Good luck. Hope this helps!
