We don't currently have a feature that allows you to add a maximum PTO cap or a feature to keep an employee from going into the negative. Both features are on our list for future development.
In the meantime, to help regulate your employees PTO you could take advantage of our PTO approval feature. To enable PTO approvals, simply visit an employees profile, select Edit, and enable both "Requires Approval for Time Card Changes" and "Requires Approval for future PTO Requests."
Whenever an employee submits a PTO request, it will show up on their Manager's or Admin's dashboard where they can review the request: