It s

unds to me that in this case when the OP acquires Spanish citizenship he's required by Spain to relinquish/renounce his us citizenship. Despite anything he may do or say to Spain at the time that won't satisfy the US government who will still consider him a US citizen.
Renouncing US citizenship is not very easy and is
rather expensive.
From the US State Dept. (linked above):
A person wishing to renounce his or her U.S. citizenship must voluntarily and with intent to relinquish U.S. citizenship:
1) appear in person before a U.S. consular or diplomatic officer,
2) in a foreign country (normally at a U.S. Embassy or Consulate); and
3) sign an oath of renunciation
Renunciations that do not meet the conditions described above have no legal effect.
I have no idea what this means with regard to Spain.