The Bitcoin.com wallet simply manages your private keys for you, and does not correspond with a "user account" in the typical sense of the word.
If you don't want to use the wallet anymore, you should make sure all funds have been withdrawn and uninstall the app.
You can also delete just a sub-wallet within your Bitcoin.com wallet, if you no longer have use for that wallet (for example a lot of our users delete their BTC wallets, because they only want a BCH wallet)
To delete a single sub-wallet just tap on that wallet from your Bitcoin.com wallet "Home" screen, and then tap on the "settings" cog in the top right of your screen. Next select "More Options" and then "Delete Wallet" (Please make sure that you read the warning. We cannot help you restore a wallet if you do not have you 12 word backup phrase)
We highly advise that users keep possession of the 12 word recovery phrase for their wallets, just in case yourself or someone you know accidentally sends bitcoins to one of the wallet addresses. If that happens, the recovery phrase is the only way to ever recover the bitcoins.
If you have purchased bitcoins through the app and created an account, that account is with Simplex, not with Bitcoin.com.
The service on https://buy.bitcoin.com is provided by Simplex.com. If you want to close your account with them you will need to contact the Simplex support team by submitting a ticket to https://www.simplex.com/submit-a-ticket/.