Let's start off with general rules:
- Don't attack their ships or colonies. This should be obvious - they're not going to forgive you while you're actively sinking every Spanish ship you come across.
I know that's hard, those treasure ships are just too juicy a target to pass up, but no.
- Don't go out of your way to help nations they're currently at war with. Same reason as before. This annoys them and will make them less likely to forgive you.
Now that you're not actively making their hatred for you any worse, let's go over the possible ways to get them to hate you less:
Wait. Bounty on your head, no matter how high, will gradually decay over time. With 60,000 dubloons on your head that's going to be a lot of time, and the fact that you can't really attack many valuable ships during that time will make it worse. It'll work, but your crew morale will be down the drain unless you do some really lucrative trading.
Give them towns. Next time you conquer a town (See general point #1, however), hand it over to a Spanish governor. They always like when that happens and will give you a couple days' head start as well as somewhat reduce the bounty on you.
Harm their enemies. Everyone's pretty mercurial; if you sink a lot of ships of their enemies while they're at war, they'll lop off a bit of the bounty. Of course, much like point #2, this will annoy said enemies, Duke or no.
Have an Abbot intercede. Depending on the amount of bounty and on how many christian artifacts you're carrying with you, it might be possible to talk to a monastery's abbot and get them to send a letter to their Spanish governor friend to maybe forgive you if you're willing to do God's work by protecting a convoy of colonists. This one's a bit hit or miss since you'd need to find the artifacts in a tavern and an abbot willing to give out that mission, but at 60K it's your best bet.
One final note: The final score only counts your highest rank among any of the nations; a quadruple duke does not get a higher point value than a one-time duke.