ive always heard that if you bless a pen then it should only be for writing spells not anything else, i've been wondering this for the past day and i hope to learn what would happen if someone misused it out of curiosity
Absolutely nothing. This is just a belief which has come about, the reason people select a pen and use it only for their spells is because it gives that pen an importance, a specific use, and in one sense 'consecrates' it to that use.
It is really a psychological effect at first, until you use it over and over and it then may be suited to that purpose, but there will be no detrimental effects if someone used it. You may wish to simply cleanse it and rededicate it.
I personally have never blessed a pen, sometimes I need to write things for rituals, any old pen does the trick for me!
You've consecrated the pen for magickal use just like any other tool, it is meant to put you in a state of 'magick'. If you or someone else uses the pen to write a letter or your homework, while some energy from the pen might be used in the writing, nothing magickal will occur. If anything, the energy will wear off and need to be reblessed. [Similar to letting other people touch your tarot deck, other energies will stick to it]