If you are not comfortable just saying "not interested" (which gets much easier after the first time), say that you have picked up another long term project, wh

If you are not comfortable just saying "not interested" (which gets much easier after the first time), say that you have picked up another long term project, which requires your full attention. If they keep insisting and saying it would only take 5 minutes of your time, counter with "I don't want to get involved in something that I cannot dedicate enough attention to". If they keep further insisting, then they are either dumb and not getting the hints or they want to take advantage of you. In either case the next thing to say - "not interested, bye". Don't fret, just say how it is. Start politely and escalate as needed.