You can change the aliasing of one or more selected peaks by right-clicking on them and selecting Set Aliasing.... You can choose which dimension to change the aliasing in, and by how many times the peaks are aliased. Note that negative aliasing will move the peak to lower ppm values while positive aliasing values will move the peak to higher ppm values. The program will then move your peaks to the new unaliased positions and repeat the spectrum contours so that the peaks are not in the middle of nowhere.
If you double-click on a spectrum in the sidebar, it will bring up the Spectrum Properties dialog. In the Dimensions tab you can set the aliasing for each dimension manually. If you don't want aliased contours to be drawn, uncheck the Draw folded contours check box. If your aliasing is such that the spectrum is mirrored rather than cyclically repeated, select Dimension is inverted.
Note that when you delete your unaliased peaks or middle-mouse button drag them back to the normal spectrum bounds, the disply will continue to show the repeated contours. You can remove these manually in the Spectrum Properties dialog by setting the Minimum/Maximum displayed aliasing back to 0. Alternatively, you can move your peaks back to 0 aliasing positions using the Set Aliasing... pop-up accessed from the right mouse menu. If you select to Update spectrum aliasing parameters, the additional contours will be removed (Minimum/Maximum displayed aliasing will be set back to 0), provided no more peaks remain outside the 0 aliasing bounds.