The following with the use of icons used for "thumbs up and down", used for here by phpbb ideas vote system, seem to me the best possible options, because they give at the admin the most hight possible elasticity in configuration of the feature ()just summarized:
Eventually in ACP there will be some fields to choose and upload the image for thumbs up / down: if the admin uploads an image for like this option only will be displayed under every post. If the admin uploads the second image for dislike option, both options will be displayed under every post.
I would like to argue about just one more thing about the label that will be present in case admin doesn't set an image icon for them.
At the same manner of icons, making option labels editable by the admin, could be the way to make appear or not the text option to vote under posts on which users can click. If a label isn't present the option won't be available.
In such way admins can decide which option to show or not just filling the related text field that will be the label of the option.
The following attachment could be the improvement that synthesize the idea for this new feature, of course they are intended to be just an example to give an idea of what I mean. Of course everything has to be optimized and adapted to style etc. In case of no icon is uploaded a special character (the triangle) could be (or not) automatically used, as well as in phpbb.com idea's vote feature.