This is true... to a point, that point being that forcing a an unhappy player to stay can have a destabilizing effect on the team (and therefor be bad for the club), but it is not true as a fact.
The club can (but would not necessarily be wise to) refuse to sell, as we did when Spurs came in for Morgan. It is a choice that comes down to what is likely to do the least damage.
*EDIT* now I see the post above...
Delaying a player leaving is the same as stopping a player leaving. With the exception MLT & Ryan Giggs, everybody leaves eventually. Nothing in football is permanent, that is why contracts, even if not stuck to are for a definitive period.