What prevents international-sports "free agency?"

Suppose Spain wanted to build a soccer superteam for the next World Cup cycle and offered multi-hundred-million-euro contracts to Mbappe, Pogba, Modric, Manzukich, Kane, etc. along with “fast-track citizenship.” It would certainly be considered extremely tasteless, but would there be anything in FIFA or IOC rules that would expressly ban such tactics?

FIFA have overturned results where players they consider not qualified play internationals.

For players that haven’t already played international football their current rules are:

The above rules, of course, only apply to players who have not played for their previous country (which would exclude all those in the original post).

Once a player has appeared in a senior, competitive game (not a friendly or youth game) for one country, the only way they can represent another country is if they involuntarily lose their previous nationality (e.g. following the break-up of Yugoslavia or the USSR)