There are a number of solutions to this. The easiest is to simply remember to press shift while selecting. Alternately, if you start to type a multi-letter command, the mouse will revert to selecting text (so you could type ':' from most anywhere, make a selection, and then press backspace to get rid of the ':').
However, if you don't like the text-based mouse handling at all, you can turn it off in the options selector (type '&', <Return>, expand "Mouse Options", change the "Use XTerm Mouse" setting, and then 'S'ave your options). This will result in this option changing in your .trn/trnrc file. E.g.:
[options] Use XTerm Mouse = No
Also note that the code that figures out the default for the XTerm Mous