I am sorry to correct you, but you have this backwards. Hadrians Wall was built to keep out the The Bru drinkers. You see, it is just too tangy, to mind-blowingly orange for a normal human too consume it and keep his sanity.
Worse still, it gave its drinkers the courage of Satan himself. These were a people not to be dealt with, and must be avoided.
For hundreds of years, we quarantined our northern brethren, lest the orange madness spread, until, on our travels we discovered the cure. (and promptly did the decent thing of invading the country it came from)
After years of trials, in the early 1700s it was deemed that they had calmed down enough to let them mingle with the rest of us.