By default, Inquisit erases all stimuli presented on a given trial after the subject has responded. Note that you can prevent a stimulus from being erased at the end of a trial by setting its erase attribute to false.
Some research tasks require that stimuli be presented for a fixed duration, after which it should be removed from the screen. To erase a stimulus after a fixed duration, you must create a separate 'blank' stimulus element and then present that blank at the appropriate interval in the stimulus presentation sequence so that it overwrites the stimulus you wish to erase.
There are a number of ways to create a blank stimulus. The easiest is to create a rectangular shape element, set its size large enough to cover the stimulus you wish to erase, and then set its color to the background color of the screen.
If you are erasing text items of various sizes, it may make sense to define a second text element that uses the same set of items, sets the select attribute as linked with the to-be-erased stimulus, but sets the txcolor and txbgcolor attributes to the background color of the screen. Each time this blank text stimulus is presented, it will present a erasing rectangle that is exactly the same size of the original text stimulus.