What a neutrality warning meant at Suez

When Britain warned Spain not to violate neutrality at Suez, was that only a diplomatic instruction or also a threat of enforcement?

Both: it was a prohibition on coaling in neutral waters and a warning that Britain could enforce the restriction.

The neutrality warning combined a prohibition with a threat of enforcement. It told the Spanish squadron to respect the restriction and indicated that force could follow if it did not. Diplomatic wording and military enforcement were both part of the warning.