From what I read, spilling of protein into the urine causes damage to the glomeruli in the kidney which eventually causes permanant damage. The sooner you can stop the spillage, the better the outcome.
I don't think you can completely stop the spillage without addressing the underlying problem (if one can be identified). A moderate-protein diet would minimize damage without causing the malnutrition of a low-protein diet. ACE inhibitors (see below) may be appropriate.
Keep in mind that the nutrition training of most vets is minimal and comes from the dog food manufacturers.
http://www.dogaware.com/health/kidneyprotein.htmlProteinuria (Excessive Protein in Urine) in Dogs
"Following are links to a series of articles and studies on the roles of protein and phosphorus in the diet of dogs with kidney disease, supporting the idea that reducing protein in the diet does not slow the progression of kidney disease nor prolong life, and is unnecessary unless it is needed to relieve symptoms of uremia (very high creatinine and BUN), or if your dog has significant proteinuria (protein in the urine, in which case moderate but not severe protein restriction is indicated)."
"There are a number of disorders associated with glomerular proteinuria including:
Immune-mediated disease (systemic lupus erythematosus)
Infectious diseases such as ehrlichiosis, Lyme disease and chronic bacterial infections
Pituitary-dependent hyperadrenocorticism (Cushing's disease)
Hypertension (high blood pressure)
Inflammatory (from circulating antigen-antibody complexes, heartworm disease)
Amyloidosis (the deposition of a body protein, amyloid A, the result of a chronic inflammatory process)
Animals with glomerular proteinuria are described as having glomerulonephritis or a glomerulonephropathy."
Diagnostic Approach to Proteinuria
"Recently, the practice of protein restriction has been questioned in certain types of GN (glomerulonephritis) in humans. It may be that higher protein diets can be fed without inducing a greater magnitude of proteinuria if they are used together with an angiotensin-converting-enzyme (ACE) inhibitor or an anti-thrombin antagonist.
ACE inhibitors have been shown to reduce proteinuria in both dogs and humans with GN. In humans, improvements in lipid abnormalities (hypercholesterolemia, hyperlipoproteinaemia) are also clearly demonstrated."