Is it possible to have more than 4 heroes or 6 nobles on the board at the same time? If not, when an Old World card requires heroes to be placed on the board, does the player just move one of the hero tokens already on the board or does he ignore the card?

You are limited to the number of OW tokens that you have, so yes.

If you run out of componetns, you just ignore the remainder of the text asking you to place. If you have less components than what is requested of the card, the player with the lowest threat decides which regions get the tokens and which don't.

but you can still take out tokens from ruined regions, or are they lying there until the game ends?

Turric4n said:

They stay on the ruined region until something instructs you to collect them.

However, corruption tokens are removed from ruined regions and returned to the respective players.

DarkPrimus said:

However, corruption tokens aren't old world tokens either.

If you're playing as Slaanesh, you should move the tokens in another region (you have a chaos cards to do it) before the region is ruined.