Quotation conversion to sales order may happen automatically on quotation win, or when triggered manually by a user, depending on your system settings.

Users with Settings access can check this from Settings - Company Setup - Company tab. The setting is Auto Convert Quotation To Order.

When auto conversion is not enabled, a won quotation can be converted to sales order by clicking on the button in the Convert column on the Quotes tab on the lead or customer record.

Administrators can restrict specific users from performing this operation

From Settings - Users - <User> - Roles, enable the Restrict Manual Conversion of Quotations role.

The convert button will still appear to the affected user but they will not be able to perform the operation.

This role will have no effect if manual conversion is enabled in company settings.