Authentication while submitting/fetching a form

Hello,

How does one enable authentication while submitting/ fetching forms in ODK
Collect from ODK Aggregate to prevent the following scenarios:

User A has valid credentials on date A, and is able to fetch and submit
forms.

On a later date B say a.the password is modified b. the user is deleted. We
find that in both cases User A is still able to submit and fetch new forms.

Thanks,
Meera