tofu

just wonder do we have the same rule here.  if there is more than certain% of owners agreed to do it, the rest of the owners are forced to chip in too.  maybe you can ask the city or someone who knows the rule more.

tofu

never run in a situation like this, but i would suggest these options:
1.  get a cat from spca, so she can survive and you can say goodbye to the rat
2.  like what yoursecret said, use the same agreement.  also, even you live an apartment, you can't tell others not to use scent product or smoke la.  
3.  pest control is under costal health, tell them what happened, what you have done, and you believe the rat problem is endanger your life already.  they will send people over to inspect your neighbour's house.  
4.  regard to the agreement, i would suggest you go to success or somewhere to get some legal advise.  if it is just a private agreement, it may not have legal authoity.