Self-service purchase gives users a chance to try out new technologies and develop solutions that ultimately benefit their larger organizations. Central procurement and IT teams have visibility to all users who buy and deploy self-service purchase solutions through the Microsoft 365 admin center. Admins can turn off self-service purchasing on a per product basis via PowerShell..

To read more about the Self-service purchase option, go to: Self-service purchase FAQ | Microsoft Docs

To disable the AllowSelfServicePurchase do the following:

#Install module
Install-Module -Name MSCommerce

#Import module
Import-Module -Name MSCommerce

#Connect
Connect-MSCommerce

#Get details
Get-MSCommerceProductPolicies -PolicyId AllowSelfServicePurchase

#Disable ProductID (or $True to enable)
Update-MSCommerceProductPolicy -PolicyId AllowSelfServicePurchase -ProductId CFQ7TTC0KP0N -Enabled $False

The following table shows the ProductID needed to enable or disable

Product	ProductId
Power Apps per user	CFQ7TTC0KP0P
Power Automate per user	CFQ7TTC0KP0N
Power Automate RPA	CFQ7TTC0KXG6
Power BI Premium (standalone)	CFQ7TTC0KXG7
Power BI Pro	CFQ7TTC0L3PB
Project Plan 1	CFQ7TTC0KXND
Project Plan 3	CFQ7TTC0KXNC
Visio Plan 1	CFQ7TTC0KXN9
Visio Plan 2	CFQ7TTC0KXN8

source: Use AllowSelfServicePurchase for the MSCommerce PowerShell module | Microsoft Docs