Simple Button

Yes, you can select only one ShareThis button and it will have “Share” written on it. Screenshot – http://screencast.com/t/4Ylvvxmxi7v2

In your ShareThis Plugin configuration menu, just select the ShareThis Horizontal button and remove the unwanted buttons and save the changes.

If you want to have your own custom button, then in that case you will have to add the code manually, which you can get from here –
http://www.sharethis.com/get-sharing/
Select “Website” as a “Platform” and follow the steps.

and

For more info about the custom buttons, please refer –
http://support.sharethis.com/customer/portal/articles/446558-custom-buttons

Also, here is the forum post link which contains the example code for custom button –
https://support.sharethis.com/customer/portal/questions/5229121-exact-code-for-custom-icon