When you say you "Can't", you are using a contraction, the long form is you "Can Not". Can, not. Another form is the compounded word, cannot. Same meaning...

You CAN , "not do it", and you CAN "do it"

SO, it is a choice...you CAN engage in the acivity of NOT doing something. That implies, since you have a choice, that you also CAN engage in that activity.

Bottom line? Each time you say you "Can't", you are also saying you "CAN!"

so, make your choice...

vitaman