If you want to suspend the account using WHM API from SSH and mention the reason of suspension then use below command.
whmapi1 suspendacct user=userame reason="Billing or Abuse complaint"
replace username with actual cpanel username.
If you want to unsuspend the account using WHM API from SSH.
whmapi1 unsuspendacct user=username
replace username with actual cpanel username.
To check cpanel account summary or want to check the reason of the suspension then you can use command.
whmapi1 accountsummary user=username
replace username with actual cpanel username.