There are multiple tools on the FortiGate that should be enabled. A quick list is below.
Enable the Botnet detection to block known crypto botnets like CoinMiner.XMRig – Under the Internet facing interface “Scan Outgoing Connections to Botnet Sites”
In your Application Control Protocol add the application overrides for the name “Cryptocurrency.Miner” and set the action to block.
To stop the JavaScript miners running on web browsers from sites that your users visit. Check your IPS profile is blocking the Generic.JavaScript.Cryptocurrency.Mining.Script.
FortiGuard also has antivirus signatures for Crypto Currency miners that you could enable if the desktop is not already providing this feature and the FortiGate has enough resources available to enable this function.
Regarding web filtering – if you don’t want people visiting the mining home pages you can use the “Web rating overrides” to re-categorize the Crypto Currency Sites from “Finance” to any other category then block that category.
Fortinet musings 