Your question is not very clear. Do you mean that you have a menu with checkbox menu items such as this? And are you asking whether one should be able to toggle on and off the "custom item" just like they'd toggle the four preset ones?
Based on that, I think the answer you're looking for depends on the relationship between the different items. Are all menu items applicable at the same time? Checkboxes are what you need then. Are they mutually exclusive? You then need radio buttons to pick which item applies. In both cases, does it really make sense to define a custom item and then turn it off? If so then it seems appropriate to display a checkbox or radio button next to the "Custom..." item.
Let's now move on to what your UI tells to the user: when you click on a checkbox menu item you expect it to turn on and off, whilst when you click on a normal menu item you expect something external to the menu to happen (such as a dialog opening). Clearly your item shouldn't communicate two different outcomes at the same time.
You should look for User Interface guidelines for the specific platform/toolkit that you're targeting. How do other applications inform users that a specific property is being enforced? Is there a different style of highlighting for currently enforced/applied items and for toggles? Does your platform use emphasis on the currently applied item, or a ✓ "check" symbol that does not look clickable to indicate that it acts merely as a feedback?
Alternatively if you feel the visual cues are ambiguous for your users, maybe you can replace "Others..." with an action verb that makes it clearer an action will occur as a result of clicking on the item: "Customise..." for instance.