If you'd like to set a deadline for accepting RSVPs to your event, you can do so from your RSVP & guest list settings You can get there by clicking the Details button from the left side of your invitation's Create/Edit Page.
In the section under Collect RSVPs, you can click to turn on the "Set RSVP deadline" toggle (blue is on). Then, enter the date and time for when you would like RSVPs to be closed:
We also recommend mentioning the RSVP-by date either on the invitation itself or in a Details block below the card so that it is clear to guests.
You can always go back and change the RSVP deadline from your Event Details if you need to.
When the RSVP deadline date is reached, any invitees who have not yet responded will see a message that reads "Sorry, RSVP is closed and this event is no longer accepting responses." and will be unable to submit a response.
If you need to remove the RSVP deadline, you can always return to the Create page and toggle the "Set RSVP Deadline" button off so it turns grey. You'll just want to make sure to click the blue Save button in the upper right corner of the page.