In my opinion this is impossible, while keeping plugin GPL-compatible (which is considered cultural norm for WP extensions, legal claims aside for now).
Much of open source licensing in general is to accommodate concept of derivative work. That is for it to really be open source other should be allowed to change it. If they change it then it's their work now however in many licenses original work still needs to be credited of course.
What you seem to want is license that is "free as in beer" but not open source.
I can't recommend one form the top of my head, but you might find some searching for free/freeware licenses.
Do note the point on GPL incompatibility. That will bar your plugin from being hosted in official WP repository (if you want to) and is rather easy way to get a mountain of negative feedback.
And if someone wants to pass your work as their work... will the license really ever stop them?