you can always un-drag the highlighted bit so it wont copy, before letting go of the button of course. If it's for highlighting text so you can read it, perhaps consider changing font/colour scheme.
One thing that does annoy me since 6.17(i think(whenever the richtext stuff went in)) is when the fore and background colours are the same you can no longer highlight the 'colour block' to see what the actual text says below it, leading to newbie troubles with
"click this block for free laptops --> 4,4http://spamaddress.com"
or whatever. Plus I just wanna know what things say, without stripping the colour from everything. I like mIRC having colours, who was it who said IRC is like multi-player notepad again? lol
btk