Not only can you prune fuchsia plants back, you should make it an annual practice. Pruning the plant will encourage continuous and healthy growth of flowers and foliage.
Benefits of pruning
In addition to promoting new growth, pruning enables the plant to bear good quality blooms. If the plant is not pruned, there is a risk that it will become "woody" and the blooms will be of lesser quality than the prior season's blooms. While it may seem aggressive to a beginner, it is actually beneficial to cutback the plant's branches by one-third. Do not worry. Flowers are produced from new growth, versus old branches. Your fuchsia will thank you and come back with vigour and a bounty of blooms.
Allow the plant to go through its normal dormancy. This will happen during the winter months.
Prune the fuchsia in the spring once all signs of frost have disappeared. Carefully remove old compost, being mindful not to shock or disturb the root system. Examine the soil for any stray, above ground roots and give them a slight trimming.
If the plant is grown indoors in a container, you can also transplant it to a smaller pot after pruning. Blend in a compost with perlite into the soil to encourage a resurgence of growth. Water sparingly so that the plant can establish itself, adapt to its new environment and soak in the nutrients from the compost.