Both the spelling with and without space are found (occasionally with a hyphen, but I would definitely avoid that) in engineering literature. Hrvatska enciklopedija uses pojasnopropusni, and so does Struna (Croatian special field terminology) - see links below. By analogy to niskopropusni/visokopropusni which are used even by those who may spell pojasno propusni I strongly recommend pojasnopropusni (i.e. without space or hyphen).