Tons of ways! You can scrape a razor blade around them and then wet sand the whole fender after, and I'm sure there are chemical options as well. The thing with using chemicals though is you have to concern yourself with the effect the chemicals will have on the plastics themselves and make sure they don't get damaged. Brake fluid removes paint but I don't know if it will have any effect on your plastics or not.
85 Tri-Zinger 60
85 ATC250SX
86 ATC250SX
87 ATC250SX
02 XR650L conversion
84 ATC 480R