Blocking on the LinkedIn app:
It is possible for you to block up to 250 members on LinkedIn app.
When you block a person your name would not be on the Who's Viewed Your Profile section of that person.
If you had un-blocked a person want to block them again immediately then it is not possible as you cannot block a member again within 48 hours of unblocking.
Even if you unblock a member that would not restore your connection if you were connected before. In case you want to connect with that person again then you would have to send them a new invitation to connect.
If you block a member then know this that their endorsements and recommendations on your profile would be removed permanently.
In case you want their endorsement or recommendation again then you would have to unblock the member first. Then send a new invitation to them and then request a new endorsement or recommendation.
You would be able to block a member currently only from your desktop computer.