So the question remains: Is there an acceptable way for a Christian to have boundaries?
When dealing with other Christian's I think that there definitely is. As part of our Christian life, we are supposed to help each other in becoming more like Christ. When a fellow believer behaves in such a way that not only is sinful or un-Christlike, but also abuses other members of The Church, I think that having boundaries and enforcing them is not only smart but biblical. The catch is that the boundaries that we create and enforce have to be biblical and christ-like themselves.
While there are many passages on this, I think the Fruit of the Spirit is a good guideline to do a quick check against.
Does the way in which I am enforcing this boundary express God's love for this person?
Does it reflect the joy I have in the Lord?
Are you expressing the boundary in a way that should promote peace?
Are you being patient with the person while enforcing the boundary? Also, are you persevering not only in making sure your boundary is Godly, but with that person as they work out their own Christ-walk?
Are you expressing the boundary with kindness, not to hurt or belittle or even accuse?
Is this boundary good and is your enforcement of it good? Do both things coincide with biblical teaching and avoid sinfulness?
Are you relying on Christ for the situation and not on your own wisdom or powers? Do you have faith that He will work things out on your behalf?
Are you being careful to express yourself in a way that is not needlessly hurtful and that is clear? Are you doing your best to make sure that your expression does not get in the way of your message? If it is possible, are you making sure to help the person feel cared for even as you are enforcing a boundary against them?
Finally, are you controlling yourself? Are you letting anger or resentment cloud your judgement?
Really, these are good guidelines for dealing with anyone, Christian or otherwise. With non-Christians, I think there is an added responsibility to sacrifice our own comfort on their behalf. They do not share our view of creation, nor are they being made more Christ-like, so we have to ask the extra question of whether this boundary is leading them to Christ?
I think a big issue a lot of us have is seeing kindness and gentleness as weakness, as well as seeing boundary enforcement or saying 'no' to requests as unkind. The strongest people are those who can be strong while also considering others; and sometimes the kindest thing you can do is say 'no' to a request when you know that saying 'no' will ultimately help the other person more than 'yes.'
Now just to figure out how to do all that all the time.