Effective way to block Playstation Store without deleting/recreating users?

I believe the best way is to create a family and not try to block individual IP addresses because you have more features on the PS4 such as internet browser and chatting which can be accessed by every user. Unfortunately you can't demote a family manager to a child account.

Setup

• A user can be in only one family at a time.
• Children are automatically added to the family of the adult who creates their user accounts. Children cannot be removed from the family they are added to, so be sure that the adult who creates a child’s user account intends to be the family manager.
• Family managers and child family members cannot leave their family.

A family may only consist of maximum 7 members, there are 3 types of accounts you can add to the family. You can add existing or create PSN accounts and add them as child or adult. The type is based on the date of birth you put in when adding a member to the family.

• Adult with parental controls
• Adult without parental controls
• Child without parental controls and with restricted access

The fourth type of account is the family manager and is created when you setup a family for the first time.

I hope this answers some of your questions.

I'm not sure if this does exactly what you want, but there are also system level controls. You can reach these by going to PS4 System Restrictions

Settings -> Parental Controls/Family Management -> PS4 System Restrictions

You'll be asked to set up a PIN.

In the "Age Level for Games" section, you can set the age area you want to restrict to. The name is misleading, because this also controls what content is shown in the store. I.e. if you set this to "13", the store won't show M rated content.

Note that since this is not by account, it will restrict anyone who uses the console.