How can I set it so that it only appears if the visitor hasn't submitted the form on my landing page?

- By setting up Custom Attributes you can set the popup up to be displays those visitors only who haven't submitted the form only. The disadvantage of this is that it starts to collect the data of who of your visitors have already submitted the form only after you insert the javascript code of Custom Attributes into the source code of your site.

Please check the following link as a reference:

http://my.optimonk.com/bundles/wseintellipopup/pdf/en-variable.pdf?v=105

- The alternative option is if you store the information of who has already submitted the form with cookies. If you have that, you can use this information with Custom Attributes as it will know who has already submitted the form before the code has been inserted to site.