You can close submissions at any time so that submitters can no longer add or edit their submissions.
Start off on your project home page
Goto Submission Settings (where Submission is the name of your submission type)
Here there are two switches you can use to control whether submitters can add or edit their submissions.
Submitters can add Submissions
Turn this switch to No to stop submitters from adding new submissions. They will still be able to edit their existing submissions.
Submitters can edit Submissions
Turn this switch to No to stop submitter from editing their existing submissions.
To fully close submissions so that submitters can not add or edit any submissions, set both of these switches to No