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What is needed is a stub filter. Just a piece of open ended coax 13" long "T"ed into the antenna lead.... The 13" is about 66% of a quarter wave at 2 meters.

That could work too, but not if they have an active splitter that's distorting and crunching away like a little fuzzbox, generating all kinds of harmonics.

In that case, the stub needs to go on the input of that device. Though, I have not seen a lot of "F" connector "T"'s... which would make it easy. So best to take a piece of cable with "F"'s on both ends, and "splce" the stub in the center by soldering carefully and sealing it all up.

Bob, WB4APR