Setting limits - defining what you say "no" to - can seem like putting yourself in a box. View it, instead, as putting the things that you're not comfortable with in boxes. You can then maneuver around them or examine them more closely if you choose.
By sequestering them, quarantining them, putting these jobs/projects/conditions under glass - you are being both honest and wise. Few situations are worse than taking a job you you don't want. Either you or the client - and probably both - will be disappointed.
So set your rules and stick to them. When an opportunity arises that is on your "no" list, be clear (with yourself, at least) that it's not the type of thing you normally do.
You can always give it a second look. Remember - you're not in the box.