make single.php behave according to category?

There’s a plugin I use that displays a certain thing beneath every single post. However, for two categories, I don’t want that thing displayed.

So I was wondering if I could put some sort of conditional into the single.php such that, if the category is x and y, then to not execute the call to that plugin.

Could that be done? What is the php code to write to do that? Also, would that slow down my server too much?
Thanks