There's three ways I know to do it (choose which one you like best):
1 - After you type any text, click 'More Options' and reduce the font size to zero
2 - Use the Draw Tool to draw a single invisible pixel
3 - Type an underscore '_' in the bottom text box, then drag the text box down until it's hidden
After you've done one of these, no text will show when you generate the meme.
