Unfortunatelly I'm not aware of any other way to fix GPUs fans 100% than replacing them.
Some can be fixed temporally using WD40, but many fans are glued together and the WD40 separates the glue and turns some plastics brittle, destroying permanetly the fan.
Maybe using a silicon based lubricant spray can help (the lubricating gel inside is silicon based), but I haven't tried it yet, can't tell if it's a good solution or even a temporally fix. By my experience not worth to fix the fans, beacause all of them die again (now for good) in a couple of months max. If you have many GPUs of the same model, maybe it's time to take working fans from the "better ones" to replace the broken ones in the most used/abused ones, then buy spares from ebay an place them in the "better ones". While you're in it you should replace thermal compound (I have a 20g MX-4 for CPU/GPU thermal paste re-aplying) and check/replace the silicon thermal pads.