Frustrating service by NOS

i have complain about broadband connection and want to discontinue but now NOS is denying to discontinue the contract instead asking fine of 600euro which was already discussed while taking subscription that if anytime I didn't like the service I can discontinue without any charge .

And the broadband connection is irritating , it's disconnect in every minutes and it's impacting my office work alot and they are sending technician every time to solve the issue but it's getting worse day by day I am frustrated now just paying the money for this useless and irritating service