The most ideal and fantastical (= improbable the devs would ever give a f***) solution, would be to re-balance polearms as a whole. Particularly, swinging polearms are just way too high in damage+way too lenient in it's handling.
Also, there's a reason why large swinging polearms weren't usually all that popular on horseback in real-life. I won't go into detail here, but swinging polearms on horseback are almost non-consequential in the history of horseback fighting. It is very commonly depicted in media that "Asian cavalry" carry around glaives (ie., the famous imagery of Guan Yu from the RotTK), but the reality is even in Asia, glaives were predominantly infnatry weapons.
The very rare, almost only, exception would be late-16th ~ early-17th century Ming heavy cavalry, whom experimented with glaive weapons to fight AGAINST increasing incursions from tribal warriors such as the Jurchens -- but this experiment was also short lived since East Asian armies rapidly transitioned to firearms with the beginning of the 1600s.
The reality is, since the earliest artifacts depicting Scythians, to the famous Companions of Alexander, the powerful heavy cavalry of the Persians, famed Celtic and Germanic cavalry serving under Caesar, the Cataphracts of the Sassanids, tjem the cataphracts of the Eastern Empire themselves, the earliest knights that fought in Hastings, the ones that roamed Europe and invaded Middle East during the Crusades, the Seljuk cavalry that fought against them during the Crusades, to the Hussars of Poland and revived lancers of the Napoleon Era... the tribal warriors of Alans, Cumans, Huns... the Central Asian Khwarezmian cataphracts that fought against the Mongols, the Mongols themselves, the Khitai, the Jurchen... the Chinese cavalry from Han to Ming, the cavalry of the Korean peninsula.. Japan.. etcetc..
They were all spear-armed. Predominantly, throughout the ages.
...
The simplest solution? Disable use of swinging polearms on horseback.