Hi Marcelo,
This can be easily changed by editing the appropriate Activity types to show/not show on your Calendars. Simply go to Customize > Screens > then highlight the CRM_ActivityTypeList screen name and click View to the left of it. You'll need to select the appropriate Activity types then enable/disable the Show on Calendar? option; check out the screenshot below. Remember this change will be made globally, so the changes will reflect on all of your Activity calendars.
Hope this helps! Let me know if you need further assistance.
- Ashur
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Ashur Shamon
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Method Integration Inc.
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