Have tried everything, can't get one of the grinding wheels off.

On a bench grinder the wheels turn towards you. This means the grinding wheel on the right side, has a right and nut, and the left side has a left handed nut. If you're trying to remove the left side turn the nut counter-clockwise

There is a left handed thread and a right handed thread on the arbors.

Double nut is the preferred method.

OR

Removal
Right Hand Side (RH)
Wrap a piece of rubber or leather around the threads on the LH side of the arbor & hold with pliers, while turning the RH nut CCW.

Left Hand Side (LH)
Wrap a piece of rubber or leather around the threads on the RH side of the arbor & hold with pliers, while turning the LH nut CW.

Installation
Note: Hand tighten the nut, then 1/4 turn.
Right Hand Side (RH)
Wrap a piece of rubber or leather around the threads on the LH side of the arbor & hold with pliers, while turning the RH nut CW.

Left Hand Side (LH)
Wrap a piece of rubber or leather around the threads on the RH side of the arbor & hold with pliers, while turning the LH nut CWW.

For grinding stones:

• ring test the stone
• discard the stone if it has been dropped
• be sure the paper washers are installed on the wheel