Daniele,
Did you look at the "disableValidators" attribute for an input control? You can use it to conditionally disable a validator.
Sample:
<xp:inputText id="inputField1">
<xp:this.validators>
<xp:validateRequired message="This field is required."></xp:validateRequired>
<xp:this.disableValidators><![CDATA[#{javascript://disable validator based on another field (note: to disable return true)
return
(currentDocument.getItemValueString("someOtherField")=="1" ?
false :
true);
}]]></xp:this.disableValidators>
</xp:inputText>