You cannot block an app that has been previously downloaded by a child:
Approve what kids buy with Ask to Buy - Approve what kids buy with Ask to Buy - Apple Support -
"If a child redownloads a purchase, downloads a shared purchase, installs an update, or uses a redemption code, the family organizer won’t receive a request."
See these tips on possibly being able to discourage use of this app by a child:
How can I cancel previously approved apps… - Apple Community
You can use Screen time to block the (previously approved) app. Screen time.. App Limits.. Add Limit.. Then pick app.. Set time limit to 1 minute (not sure why 0 minutes, doesn't work, but it doesn't), then set to "Block at end of Limit".
Block a Child's App that's already been d… - Apple Community
The only work around I have found this far is to set the screen time limit on that app to 1 min, then they will have to request more time to continue using it. It’s not ideal but it seems to do the trick for now, unless you think they can get into trouble in 1 Min time.
How do I permanently delete social media apps from a child's iPhone?
How do I permanently delete social media … - Apple Community
Post about toggling the downloading of ANY AND ALL apps by a child if there is one they have already downloaded:
How can I stop Roblox from re-downloading… - Apple Community
Includes providing feedback to Apple about the lack of a way to disable redownloading a once downloaded app.
Yours is not the first post I have seen about this. Using the feedback links, suggest to Apple it adds this feature, --> Product Feedback - Apple Apple may not respond but your comment will be read.