Try a strip of double-sided sticky tape like carpet tape and be brought in a wide tape..... they won't like that. Make sure the sill is clean and dry before applying.
Cats LOVE windows and it is their only way for them to see outside...if they are indoor cats. As a cat lover I do not understand why you would deny your cat to sit in the window. This is only a suggestion but has worked well for me. Purchase a window seat for cats and they will love you forever. If there is one certain window you do not want your cat to jump on, you can use a spray bottle and squirt him/her while 'they' are up there (this is important) and say in a stern voice, "No." After a few times the cat will get the message. Oh, by the way cats do not like a citrus smell. Good luck!