Under the Desktop Pool Settings tab in the 'Remote Display Protocol' section you have the ability to set the 'Default Display Protocol' to PCoIP. There should also be another option 'allow users to choose protocol' which controls whether users can override the default display protocol you selected. Simply change that value to No and it will force them to use the default protocol which you set to PCoIP. There is also a Group Policy setting you can set called 'AllowDirectRDP' which will prevent devices that are not running Horizon Client from connecting directly to Horizon Desktops through RDP: (Computer Configuration > Policies > Administrative Templates > Classic Administrative Templates > VMware Horizon Agent Configuration) and disable 'AllowDirectRDP'
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