Improper pruning can shorten a tree's life. However you would have to work really hard to damage a willow with bad pruning. Here's why:
- Willows grow fast
- they will bud from old wood, even really old wood
- they are a messy tree, commonly dropping leaves and branches
- they are used to breaks or cuts in the branch structure
- willows have been coppiced for centuries. If that kind of hard pruning doesn't kill them off bad pruning is not likely to either. See here for a detailed discussion on pruning for look with willows
If you continue to use a hedge trimmer at the seven foot level you will cause the tree to develop a mass of new growth at that height. It would be better practice and more aesthetically pleasing to use a pole pruner to deliver a series of graduated cuts at various heights. This will yield a wide band of new growth instead of a ribbon at one height.
Edit: @David You are right. Trimming with a hedge trimmer is essentially pollarding. However all the cuts are at the same height and, if continued for years, will produce a thick mass of branches. Traditional pollarding creates quite a different look.