A new boolean has been added which allows to suppress adding the default signature when calling the ox.api.mail.compose function.

Just add the option to the data object. For example:

ox.api.mail.compose({
data: {
default_signature: false,
...
}
});

When set to false the default signature won't be automatically added to the message text.

Please note: The referenced API is still in progress and marked as draft. It may change in the future!