While the broader issue of permissions is covered elsewhere, we want to take a moment to note one area of primary concern when working with contributions. We strongly discourage deleting financial data transactions as these records have tax- and fiscal-responsibility-related implications. You can disable the delete in CiviContribute and delete in CiviPledge permission from all roles or access control groups to prevent intentional or unintentional deletions.
Review the permissions related to contributions by visiting Administer | Users and Permissions | Permissions (Access Control). The first section on that page will provide a link to the respective CMS permissioning tool where you can modify access.