It should be possible to write a cookie that would, for example, disable the submit button if the IP of the visitor has made a submission in the last 12h/24h/72h etc.
However, you should be aware that blocking by IP address might have adverse effects depending on your target market. (ex. People in the same office sharing a connection would be blocked for the pre-set amount of time).
If you really need something like the above scenario, you can reach out to me and my company should be able to write it for you.
Also, in theory, you should be able to use a webhook to push your lead’s data to a google spreadsheet. Using that spreadsheet as a DB and querying it with a bit of frontend scripting. If the email is in the spreadsheet/DB, alert the visitor and ask them to provide another one.