Vinyl lettering can be peeled off - use a hairdrier/heatgun/hot water to soften the adhesive, if it proves difficult. Clean the residual adhesive off the transom with solvent - gunwash is good.
Paint will be more difficult - it may scrape off with relative ease, if the transom surface wasn't keyed before the paint applied. Use paint stripper with caution, as it may damage the gelcoat. If you go down the scraper route then you're likely to have little remnants of paint left behind. Again, solvent is good for getting rid of this.
Don't use a blowlamp.
Edit: You could always use the phone number to contact the original owner to ask whether it's paint or vinyl lettering.