I believe that when a user action is done on the editable ALV,if the field is referenced using REF_FIELDNAME and REF_TABNAME , then a popup will be displayed when some user action like Save,Double click etc is encountered informing the user that a wrong type of data has been entered.As far I have used,thereis no actual restriction for the value being entered in the editable field in REUSE_ALV_GRID_DISPLAY.There may be options in the Object oriented version of the ALV though(not yet explored).
In any case we can check for Strings or numerics or special characters entered by user with "Relational Operators" like CA(Contains Any) , CO ( Contains Only) , CS..Etc with the respective field and throw error message to users when some user action is encountered.
Hope it helps.