Tactics to fight or deter Diplomatic races in 2 player games
As @bwarner commented, I'd say abandon all regions and start invading from the maps borders

From the rules (emphasis mine):


  If a player wishes to free up some more Race tokens, he may opt to entirely empty up some - or all - Regions, leaving no Tokens there; but in this case, these now abandoned Regions will no longer be considered his, nor bring him any Victory coins. If the player chooses to abandon all the Regions he previously occupied, his next conquest must follow the same rules as its First Conquest (see First Conquest, p. 4).