For Watson, probably not "trivial", no, but also probably not a hugely expensive undertaking. It really depends on how the system works, though. Something like an artificial neural network would be impossible to manually prune like that; they'd have to retrain it and "teach" it that those things are bad. Without knowing much about Watson, though, my guess would be that its knowledge is largely stored in a structured database, which is more directly accessible.

Watson's 'memory' is just a big database of facts, rules, and statistical models. To 'forget' a source they'd just have to rebuild any models derived from it and purge any facts it had extracted.

they just overwrote every urban dictionary word instances with the string "rainbow". So Watson still wants to call the query bullshit, but says rainbow instead.