See the Explanation for the complete step by step solution.
Explanation:
To prevent all users from using legacy authentication protocols when authenticating to Microsoft Entra ID, you can create a Conditional Access policy that blocks legacy authentication. Here's how to do it:
Sign in to the Microsoft Entra admin center:
Ensure you have the role of Global Administrator or Conditional Access Administrator.
Navigate to Conditional Access:
Go to Security > Conditional Access.
Create a new policy:
Select + New policy.
Give your policy a name that reflects its purpose, like "Block Legacy Auth".
Set users and groups:
Under Assignments, select Users or workload identities.
Under Include, select All users.
Under Exclude, select Users and groups and choose any accounts that must maintain the ability to use legacy authentication.It's recommended to exclude at least one account to prevent lockout1.
Target resources:
Under Cloud apps or actions, select All cloud apps.
Set conditions:
Under Conditions > Client apps, set Configure to Yes.
Check only the boxes for Exchange ActiveSync clients and Other clients.
Configure access controls:
Under Access controls > Grant, select Block access.
Enable policy:
Confirm your settings and set Enable policy to Report-only initially to understand the impact.
After confirming the settings using report-only mode, you can move theEnable policytoggle fromReport- onlytoOn2.
By following these steps, you will block legacy authentication protocols for all users, enhancing the security posture of your organization by requiring modern authentication methods. Remember to monitor the impact of this policy and adjust as necessary to ensure business continuity.

Reference:
https://docs.microsoft.com/en-us/azure/active-directory/governance/create-access-review

Reference:
https://docs.microsoft.com/en-us/azure/active-directory/hybrid/choose-ad-authn