"Dismark" is not a standard English word.
Here's why:
* Un- is a common prefix in English that means "the opposite of." So, "unmark" means "to remove a mark."
* Dis- can also indicate an opposite, but it's often used for actions that imply separating, removing, or undoing. While "dis-" might seem like it could fit, it doesn't have the same specific meaning as "un-" in this context.
Examples:
* "Please unmark the items you don't want."
* "The teacher unmarked the student's paper after realizing it was graded incorrectly."