I always used a combination of bullet-proof written penalty text, and very lax actual timing:

Official text: The submission system guarantees acceptance until 31. Nov 1pm. The submission system will close after the deadline and late submissions will not be eligible to any points.

Then configure the system to close down 24 hours later. Anyone who has not submitted by then will be more than 24 hours late and have no grounds to argue. - Anyone a few hours late will thank god for his luck that submissions were still accepted. You don't have any hassle.