How can I remove the substitution?

You can directly deleted only those lessons, which were manually created in the substitution or lessons you move. Simply do "Right click" and choose "Remove substitution":

The substitutions lines that are created because someone was absent cannot be simply releted. If some teacher is missing and he is teaching certain lesson, you have to do something. Either cancel this lesson, or assign another teacher.

The only way how to remove the substitution is to remove the original missing teacher from the list of absent teachers (or adjust the length of absent if he/she is missing on part of the day only).