Recently I had cable tv as well as internet coming into the home. I cut the cord regarding tv service but still want the internet feature. I want to hook up an antenna for tv. I hooked up an antenna and I tried connecting to a coax box in the house to distribute the antenna signal throughout the home but I get the previous cable subscribers error message on my tv screen telling me I need a cable set top box. I have gone through the tv menu and set it to air instead of cable. I told the cable company that I want them to cut the tv feed so I get internet only through the line. They say they can't do that. Is there a way for me to filter the line so I can use the existing line for my tv antenna??
No, you will need to isolate the two systems from each other. They cannot exist on the same coax.
Unplug lines one by one from the splitter until you locate the modem. That line must be connected to the incoming cable from the cable company. You may need an inexpensive F-Type Coaxial Cable Connector from the big box store to join the two cables.
Important note: both systems do need to be grounded. Without more information on your current system it's hard to advise. The ground in this case has multiple functions, but is primarily grounding static charge, providing ground reference, and possibly preventing a surge from damaging equipment.
Once the modem is isolated and connected directly to the incoming cable, then you can use the rest of the existing distribution system to distribute your OTA TV to the TV's.