Geneos - How to restrict user from reading the file monitored by FTM plugin?

There is a way to limit this via Authentication roles in the gateway; you will just need to add a specific permission that pertains to the 'View file' and "Accept File Delay" commands via the plugin's metric view.

<role name="Restrict FTM viewFile">
    <permissions>
        <permission>
            <command>
                <targets>
                    <target>/geneos/gateway/directory/probe/managedEntity/sampler[(param("PluginName")="FTM")]/dataview/rows/row/cell</target>
                </targets>
                <names>
                    <name>/PLUGIN:FTM:viewFile</name>
                </names>
                <access>none</access>
            </command>
        </permission>
    </permissions>
    <users>
        <user ref="support"></user>
    </users>
</role> 

You can use this with your other existing roles but please ensure that your current 'combineMode' setting for Authentication is set properly as it can either inherit the highest possible permissions or the lowest. Please see the Authentication section of our documentation for more details.