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.
I didn't mean to imply it was simple, just that there's nothing magic about how Watson's knowledge is stored. Obviously at this scale any change is unlikely to be trivial.
Given the wide range of unstructured sources Watson uses, and given that the linguistic rules they use to extract facts are likely to frequently change, I don't think it's unreasonable to assume they'll have a process to make building its knowledgebase and models from sources fairly straightforward.