I need to set up a workflow in a sharepoint calendar using the 'accept/reject' ability. I need to set this up without using sharepoint designer if possible.
I need for a user to add an event, then sharepoint email approver(s), the approver(s) then approve or reject, then sharepoint sends an email to the user informing them of the accept or reject. I also need it to not allow the user to set up an event
if there is already one there for that time slot.
If that won't meet your requirements, I think you'll need to use SharePoint designer to build that fits your requirements. Not sure if any other options available without using SPD.
As for not allowing users to double book (prevent creating an event if one is created for that time slot), I suspect you'll need to implement a custom solution for that. What you're asking for here is more of a functionality of Exchange server. Here's couple
of different approach you can take a look: