Around 1625 the spanish leader gives up all his colonies and the English take over. I get a message about the war in spain and that the spanish have given up.

Now i wondered what exactly triggers this event? I would like to play along with all the three rival nations. Is there a way to prevent this event? Thanks for any input

Sadly it can't be avoided unless you don't want to win the game. Whenever the human player reaches 50% rebel sentiment (which is the minimum to declare independence), the War of the Spanish Succession event triggers and one AI retires from the New World, leaving all their colonial possessions to another AI.

This is my least favorite mechanic in the game. :( It was included because the game's engine only supports four players (human + 3 AIs), and it needs to make room for the Royal Expeditionary Force (your King's troops) in the independence war.

Apparently you are playing the crappy older stand alone version of Colonization.
The better version is the updated newer one included with CivIV complete which does not have that feature.

Of course they are, this is the forum for Sid Meier's Colonization (1994, DOS). "Better" is a highly subjective term in this case.

Civ4:Col does some good things well but many others felt like a downgrade. Some claim that there are mods that fix it, bur I haven't tried them yet.

Thank you very much. I wasnt aware that the rebel sentiment was the key trigger, i wondered a lot, but now i know. Thanks! So at least one can prolonge the event a bit by having an eye on the total rebel score.

Keep in mind it's practically an inevitability; as even without actively placing units to generate bells, your cities will slowly trickle them out.