A All the evidence suggests that the best possible response to freshly painted grafitti is to cover it up as soon as possible. It's likely that the "artist" will strike again, and when they do, the right response is to paint straight over it once again. If you haven't already done so, make sure you have a supply of paint that matches the colour of the wall so you're ready, if possible, to destroy the graffiti the morning you notice it.
This sounds like a tedious game of cat and mouse, and it can be just that. But in most cases the vandal will eventually give up, and pick a target where their handiwork will remain unmolested.
Local authorities will often deal with graffiti on residential property facing a public highway and remove the scrawl either by painting over it, or with solvents or pressure washing. But for business premises, you're effectively on your own unless the council has embarked on some sort of crackdown.
It sounds like you need to find ways of nipping this problem in the bud. It's possible to buy anti-graffiti paint, though it's not cheap - expect to pay over £100 for five litres. There are plenty of suppliers accepting online orders, such as antigraffitipaint.co.uk and astroflame.com, and matt and gloss finishes are available.
You could also consider pointing a CCTV camera at the wall. A dummy camera might do the trick.
Another option might be to put up a trellis and grow climbing plants along the wall. Evergreen clematis could be a good year-round bet, but vines and hops would create a nice effect.