By standard checkbox do you mean a standard field? If you have a custom or extension controller you can simply change the value on the record before saving it. If you are using a standard controller you'll need to add some JavaScript - I would add some to set the input to checked on load of the page and then blur every time the user tried to focus on it, which would have the effect of stopping them from changing it.
Thank Bob On the custom edit form the field (All-Day Event) is unchecked but once the form is saved it is checked by default, no triggers or no workflow rules are written, how can this be done
On the custom edit form the field (All-Day Event) is unchecked but once the form is saved it is checked by default, no triggers or no workflow rules are written, how can this be done
https://developer.salesforce.com/docs/atlas.en-us.api.meta/api/sforce_api_objects_event.htm
https://developer.salesforce.com/docs/atlas.en-us.api.meta/api/sforce_api_objects_activityhistory.htm
It might be taking default value(True) on create as you are not supplying any value. Did you try setting this as false in code as Bob mentioned?