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Multi-selects are store as tab separated values with a leading and trailing tab. For example, A, B, C would be stored as: (tab)A(tab)B(tab)C(tab)

In Mysql and PHP you can refer to a tab with \t

So to see if a multi-value list contains an option you could use:
category LIKE "%\tA\t%"

To return all records but those with BOTH A and B selected:
'where' => 'category NOT LIKE "%\tA\t%" AND category NOT LIKE "%\tB\t%" ',

And sometimes it's easier to group things with parentheses and to put the NOT in front.

So instead of: "NOT A and NOT B" you can say: NOT (A AND B) or using the above example:
'where' => ' NOT (category LIKE "%\tA\t%" AND category LIKE "%\tB\t%") ',

Hope that helps!