Decimal places are shown in a Currency Field even though Decimals is set to Zero in the field settings

This is an expected behavior when Multi-Currency is enabled in your Org. Settings to a currency field for an object will be superseded by the setting in the Currency setup
 
For example: (Consider the Decimal Place setting on the currency field is set to 2) 

1. Type the opportunity amount as “123.89” on the record and its Currency field Decimal Place setting is set to “0”, the amount will display as “124”. 

2. Similarly, entering the opportunity amount as “124” on the record and its Currency field Decimal Place setting is set to “2”, the amount will display as “124.00”, irrespective of what the Decimal Place setting is defined at the field level.

Locating or Editing 'Decimal Place' Settings at the Currency Level

Salesforce Classic:

1. Click on your Name | Setup | Administration Setup | Company Profile | Manage Currencies (If you have “Enable Improved Setup User Interface” disabled). OR Click on your Name| Setup | Administer | Company Profile | Manage Currencies (If you have “Enable Improved Setup User Interface” enabled).
2. Click on the button ‘Manage Currencies’ which will take you to the Currency Setup Page.
3. Under ‘Active Currencies’ section, you can review the ‘Decimal Places’ defined for individual Currency.
4. To make changes to the same, click on ‘Edit’ next to the Currency you want to make changes in.

Lightning Experience:

1. Click the gear icon, Setup | Company Settings | Manage Currencies, then click Manage Currencies button.
2. Under Active Currencies, Click Edit next to the Currency that you want to change or update the decimal places.
3. Click Save.

SEE ALSO:
Manage Multiple Currencies