In MetaMask Extension:
To remove a token, click on the token. You will then see a 3 vertical dots icon in the top right. Tap on it, and it should bring up a dropdown with a Hide Token option:
On MetaMask Mobile:
To remove tokens from your asset list in MetaMask Mobile, simply hold your finger on the asset you want to remove and it will bring up the option to remove or cancel:
Please note that this action does not affect the balance of tokens held. It is simply removing the listing from displaying in your wallet; so if you did have a balance of DAI, for example, and removed the listing on the MetaMask interface, it would not remove the tokens from your address on the blockchain.