I do not know about blocking the gap junction but you can take indirect help from articles on "different dyes passing thru different pores" e.g., Selective permeability of gap junction channels
Selective dye and ionic permeability of gap junction channels formed by connexin45. R D Veenstra et al
I am not sure if something useful can come out from this direction.
Yes, it is possible. You can run a search with the words "downregulation of Cx45" and you will find several articles on the issue. I like the Circulation Research paper of Alcolea et al. from 1999. However, if you mean to specifically and quickly block Cx45 in normal cells expressing also Cx36, you will learn that a "pharmacology of gap junctions" is still underdeveloped. Take a look at papers exploring the function of "connexin mimetic peptides".