They need a renewal pruning. If you aren't up for reading the whole documnet attached, scroll down to the heading "Pruning Overgrown Shrubs". Late January early February is usaully recommended for renewal pruning, but you could do it with ligustrum now and they would flush back out in spring.

Here is a link that might be useful: renewal pruning

Frankly, ligustrums are invasive plants in the south. Usually planted by birds these days, they were used years ago as hedge plants. I would encourage to get rid of some and plant other things (wax myrtle is a great native evergreen plant for screening) and as those other things fill in, cut more out and so forth. At the very least, keeping them trimmed so they don't set seed would be helpful.

Anyway, they are quite tough as nails. If you cut them to the ground, they will spring back in no time.

Yep, I had 2 in my front planter and had a devil of a time trying to keep them pruned. Found out they were invasive and that's all it took to pull them out of the ground and replant something less aggressive. But, it wasn't easy getting them suckers out of the ground. Had to chain them up to my pickup and almost pulled up the sewer lines with them. Whew!! Those boys got some roots.

Personally , I am not a huge fan of Wax Myrtle . It is too spindly , scrawny and thin for my likes and does not make as full a hedge as other choices out there . At least that is what I have seen here in Florida .
Ligustrum , viburnum or any large plant - the rule is cut only 1/3 at a time .
The thinking is that any more will put the plant into shock and you will have total die back .
Pruning spurs new growth which you do not want with winter coming soon .
Prune in the early Spring .
Always prune in a pyramid shape to encourage lower growth fill -in .
Allow the sun to hit the lower leaves to encourage growth there .
idig - thank you for posting the excellent link on pruning .

Being the "thug", that it is, you can prune, butcher, hack it to within an inch of the ground, anytime of the year and Ligustrum will "bounce back" with renewed vigor.
If you are going to maintain it at 6-8' height, get out the chain saw, Hedge Hog and a sturdy step-ladder and go to work! Now is as good a time as any to give it a haircut.

A former neighbor planted a row along the property line, but mistakenly planted it ON the property line, so I claimed half ownership. They maintained it at about 4'. The new owners want it to grow to astronomical heights, so 2 or 3 times a year, I get out the Hedge Hog and cut it back to 4-5', amid yelling and threats from them. Every time they complain, it just gets shorter. Guess it reminds them of the Jungle, from whence they came.
It was originally void of foliage near and within 2' of the ground, but 2 nights of -12°F on Jan 21-22, 1985, killed it and all other shrubbery back to the roots. Tapered pruning of the new growth that followed, allowed sunlight to reach the lower growth and today, it is full all the way down to near the ground and about to receive it's second haircut this year, back to the 4' height!. My pleasure! Will also rid it of the berries that birds love.
Rb