Hydrangea Scale

Looking for advice on what to do with my mop head Hydrangea - it's about 7 years old, planted in the front garden, not in a pot.  The stems and underside of the leaves (which have black marks on them) are badly covered with scale and it has not bloomed this year at all.  Last year it also had scale and hardly bloomed.  I sprayed it last year but admit I've neglected it this year and feel bad about it now.  Is it likely that this will be a recurring problem, in which case I could dig the plant up now and maybe replace it with a new hydrangea in the spring?  Or is it worth cutting it right back (to the ground?), in which case when is the right time to do so?