Actually, it's not that bad that it shows thank as your biggest word. It shows that you're a grateful person.

Moreover words like "think" and "believe" could be interesting for writers. If they pop up too much on the word cloud, the writer needs to pay attention to use them less, because they make one's writing weaker.

So, this can be a parameter for the user. Just an empty text box, with the option to populate it with common words like you mention. And the user could have the option to add extra words to be excluded. Eventually, this text box could be prepopulated with a standard list.