there is a way that works
Found an interesting great workaround to keep players at their current clubs for those that want to keep say Kane at Spurs, I posted earlier in thread if you set up a future transfer that will work,
but if that is not for you , it also looks if you edit the date the player joined the club to be several years ahead/and same for date contract signed he will even if his club accept bids for him not agree to join other clubs even if they are bigger ones as he will say he has only just joined his current one
I am also using an in game editor to keep changing those dates when a transfer window is coming up, so just experimenting with this,the results are great so far ...even young/old players at lower clubs refuse contracts at big clubs
can confirm this works like a dream even if his club accepts a bid for him he always rejects the contract so becomes ''untransferable''